I'm trying to get Apothic Invaders to spawn naturally but they aren't spawning after increasing world tier etc. I even set I:"Boss Spawn Cooldown"=1 in the Apotheosis config which should allow them to spawn. Mod list below, I don't think anything here should interfere with Apotheosis spawning though.

Mods:
[✔] addonslib-neoforge-1.21.1-1.3
[✔] ae2wtlib-19.2.2
[✔] amendments-1.21-1.2.24-neoforge
[✔] Apotheosis-1.21.1-8.2.1
[✔] apothic_amendments__enchanting-NF1.21.1-0.1.4
[✔] ApothicAttributes-1.21.1-2.7.0
[✔] ApothicEnchanting-1.21.1-1.3.2
[✔] ApothicSpawners-1.21.1-1.2.1
[✔] appleskin-neoforge-mc1.21-3.0.5
[✔] appliedenergistics2-19.2.8
[✔] Applied-Mekanistics-1.6.2
[✔] Aquaculture-1.21.1-2.7.14
[✔] aquaculturedelight-1.2.0-neoforge-1.21.1
[✔] architectury-13.0.8-neoforge
[✔] ars_creo-1.21.1-5.1.0
[✔] ars_elemental-1.21.1-0.7.3.0
[✔] ars_nouveau-1.21.1-5.8.1-all
[✔] ars_ocultas-1.21.1-2.1.0
[✔] arsdelight-2.1.5
[✔] artifacts-neoforge-12.1.5
[✔] balm-neoforge-1.21.1-21.0.39
[✔] betterbeds-1.4.0
[✔] BiomesOPlenty-neoforge-1.21.1-21.1.0.8
[✔] buildinggadgets2-1.3.8
[✔] charginggadgets-1.14.1
[✔] chat_heads-0.13.17-neoforge-1.21
[✔] Clumps-neoforge-1.21.1-19.0.0.1
[✔] Cobblemon_MegaShowdown-9.1.0-release-neoforge
[✔] cobblemonchallenge-neoforge-2.1.0
[✔] cobblemonextrastructures-1.21.1-1.0.0
[✔] CobblemonMoveInspector-neoforge-1.2.0
[✔] Cobblemon-neoforge-1.6.1+1.21.1
[✔] cobblemon-spawn-notification-1.6-neoforge-1.0.1
[✔] cobblenav-neoforge-2.2.0
[✔] cobbleride-neoforge-0.2.2+1.21.1
[✔] cobweb-neoforge-1.21-1.3.3
[✔] CodeChickenLib-1.21.1-4.6.0.521
[✔] combat_structures_update-1.6.0-neoforge-1.21.1
[✔] comforts-neoforge-9.0.3+1.21.1
[✔] configured-neoforge-1.21.1-2.6.0
[✔] Controlling-neoforge-1.21.1-19.0.5
[✔] cookingforblockheads-neoforge-1.21.1-21.1.12
[✔] coroutil-neoforge-1.21.0-1.3.8
[✔] corpsecurioscompat-1.21.1-NeoForge-2.2.2
[✔] corpse-neoforge-1.21.1-1.1.5
[✔] craftingtweaks-neoforge-1.21.1-21.1.5
[✔] create_rustic_structures-1.0.0-neoforge-1.21.1
[✔] create_structures_arise-153.26.25-neoforge-1.21.1
[✔] create-1.21.1-6.0.4
[✔] create-confectionery1.21.1_v1.1.1
[✔] CreativeCore_NEOFORGE_v2.13.2_mc1.21.1
[✔] CreeperOverhaul-neoforge-1.21.1-4.0.6
[✔] cristellib-neoforge-1.2.8
[✔] cupboard-1.21-2.9
[✔] curios-neoforge-9.4.2+1.21.1
[✔] deeper_additional_structures
[✔] deeperdarker-neoforge-1.21-1.3.4
[✔] dummmmmmy-1.21-2.0.7-neoforge
[✔] DungeonsArise-1.21.x-2.1.64-release
[✔] DungeonsAriseSevenSeas-1.21.x-1.0.3.2-neoforge
[✔] emi_enchanting-0.1.2+1.21+neoforge
[✔] emi_loot-0.7.5+1.21+neoforge
[✔] emi-1.1.22+1.21.1+neoforge
[✔] enderio-7.1.8-alpha
[✔] endersdelight-neoforge-1.21.1-1.1.0
[✔] EnderStorage-1.21.1-2.13.0.191
[✔] ends_delight-2.5.1+neoforge.1.21.1
[✔] entityculling-neoforge-1.7.4-mc1.21
[✔] extra-mod-integrations-all-neoforge-1.0.2+1.21.1
[✔] FarmersDelight-1.21.1-1.2.7
[✔] FarmersStructures-1.0.1-1.21.1_neoforge
[✔] fastpaintings-1.21-1.2.15-neoforge
[✔] FastSuite-1.21.1-6.0.2
[✔] ferritecore-7.0.2-neoforge
[✔] fightorflight-neoforge-0.7.7
[✔] forbidden_arcanus-1.21.1-2.5.15
[✔] ftbbackups2-neoforge-1.21-1.0.28
[✔] ftb-essentials-neoforge-2101.1.6
[✔] ftb-library-neoforge-2101.1.12
[✔] fusion-1.2.7-neoforge-mc1.21
[✔] fzzy_config-0.6.9+1.21+neoforge
[✔] geckolib-neoforge-1.21.1-4.7.5.1
[✔] GlitchCore-neoforge-1.21.1-2.1.0.0
[✔] guideme-21.1.8
[✔] handcrafted-neoforge-1.21.1-4.0.3
[✔] harvest-with-ease-neoforge-1.21-9.4.0
[✔] homebound-1.21.1-1.1.0
[✔] ImmediatelyFast-NeoForge-1.6.5+1.21.1
[✔] ImmersiveEngineering-1.21.1-12.1.1-186
[✔] Incendium_1.21.x_v5.4.4
[✔] inventorysorter-1.21-24.0.20
[✔] iris-neoforge-1.8.8+mc1.21.1
[✔] ironchest-1.21-neoforge-16.0.7
[✔] Jade-1.21.1-NeoForge-15.10.0
[✔] JadeAddons-1.21.1-NeoForge-6.1.0
[✔] JAOPCA-1.21.1-5.0.8.10
[✔] journeymap-neoforge-1.21.1-6.0.0-beta.44
[✔] justdirethings-1.5.5
[✔] konkrete_neoforge_1.9.9_MC_1.21
[✔] kotlinforforge-5.7.0-all
[✔] laserio-1.9.11
[✔] LeavesBeGone-v21.1.0-1.21.1-NeoForge
[✔] lithium-neoforge-0.15.0+mc1.21.1
[✔] lootintegration_wda-1.6
[✔] lootintegrations_yungs-1.3
[✔] lootintegrations-1.21-4.3
[✔] lootr-neoforge-1.21-1.10.35.91
[✔] mcwbiomesoplenty-neoforge-1.21.1-1.2
[✔] mcw-bridges-3.0.0-mc1.21.1neoforge
[✔] Mekanism-1.21.1-10.7.13.78
[✔] MekanismAdditions-1.21.1-10.7.13.78
[✔] MekanismGenerators-1.21.1-10.7.13.78
[✔] MekanismTools-1.21.1-10.7.13.78
[✔] melody_neoforge_1.0.10_MC_1.21
[✔] mes-1.3.5-1.21
[✔] mininggadgets-1.18.6
[✔] mns-1.0.8-1.21
[✔] modelfix-1.21-1.10
[✔] modernfix-neoforge-5.21.0+mc1.21.1
[✔] modonomicon-1.21.1-neoforge-1.114.1
[✔] moonlight-1.21-2.18.5-neoforge
[✔] MouseTweaks-neoforge-mc1.21-2.26.1
[✔] mss-1.2.8-1.21
[✔] MyNethersDelight-1.21-1.7.8
[✔] mysticaloaktree-1.21-1.12-neoforge
[✔] NaturesCompass-1.21.1-3.0.3-neoforge
[✔] netherportalfix-neoforge-1.21.1-21.1.1
[✔] nnp_easy_farming-1.21.1- NEOFORGE-0.2.8
[✔] noisium-neoforge-2.3.0+mc1.21-1.21.1
[✔] Nullscape_1.21.x_v1.2.11
[✔] observable-5.4.3
[✔] occultism-1.21.1-neoforge-1.179.3
[✔] oceansdelight-neoforge-1.0.3-1.21
[✔] OctoLib-NEOFORGE-0.5.0.1
[✔] owo-lib-neoforge-0.12.15.1-beta.3+1.21
[✔] packetfixer-neoforge-2.1.0-1.21-to-1.21.3
[✔] pamhc2crops-NEOFORGE-1.21.1-1.0.0
[✔] pamhc2fantasy-NEOFORGE-1.21.0-1.0.0
[✔] pamhc2foodcore-NEOFORGE-1.21.1-1.0.0
[✔] pamhc2foodextended-NEOFORGE-1.21.1-1.0.0
[✔] pamhc2trees-NEOFORGE-1.21.1-1.0.0
[✔] Patchouli-1.21-88-NEOFORGE
[✔] pet-your-cobblemon-1.3.3
[✔] Placebo-1.21.1-9.7.1
[✔] plushed-1.0.6-neoforge-1.21.1
[✔] plushies-neoforge-1.5.1
[✔] pneumaticcraft-repressurized-8.2.12+mc1.21.1
[✔] polyeng-0.4.1
[✔] polylib-2100.0.3-build.172-neoforge
[✔] polymorph-neoforge-1.0.9+1.21.1
[✔] productivebees-1.21.1-13.6.15
[✔] PuzzlesLib-v21.1.33-1.21.1-NeoForge
[✔] rarcompat-1.21-0.9.5
[✔] rctapi-neoforge-1.21.1-0.11.0-beta
[✔] rctmod-neoforge-1.21.1-0.14.3-beta
[✔] rechiseled-1.1.6a-neoforge-mc1.21
[✔] rechiseledcreate-1.0.2-neoforge-mc1.21
[✔] reeses-sodium-options-neoforge-1.8.3+mc1.21.4
[✔] relics-1.21.1-0.10.7.4
[✔] reliquified_twilight_forest-1.21.1-0.4
[✔] resourcefulconfig-neoforge-1.21-3.0.9
[✔] resourcefullib-neoforge-1.21-3.0.12
[✔] roots-4.0.0.6-alpha
[✔] saturn-mc1.21.1-0.1.5
[✔] Searchables-neoforge-1.21.1-1.0.2
[✔] smallships-neoforge-1.21.1-2.0.0-b2.1
[✔] SmartBrainLib-neoforge-1.21.1-1.16.7
[✔] snowyspirit-1.21.1-3.0.16-neoforge
[✔] sodium-extra-neoforge-0.6.0+mc1.21.1
[✔] sodium-neoforge-0.6.13+mc1.21.1
[✔] sophisticatedbackpacks-1.21.1-3.24.7.1223
[✔] sophisticatedcore-1.21.1-1.3.25.948
[✔] Storage Drawers-neoforge-1.21-13.8.5
[✔] Structory_1.21.x_v1.3.10
[✔] structureessentials-1.21.1-4.7
[✔] structures_tweaker-1.21.1-NeoForge-1.2.1
[✔] supermartijn642configlib-1.1.8-neoforge-mc1.21
[✔] supermartijn642corelib-1.1.18a-neoforge-mc1.21
[✔] supplementaries-1.21-3.1.6-neoforge
[✔] t_and_t-neoforge-fabric-1.13.2
[✔] takesapillage-neoforge-mc1.21.1-1.0.6
[✔] TerraBlender-neoforge-1.21.1-4.1.0.8
[✔] ToastControl-1.21-9.0.0
[✔] trashslot-neoforge-1.21.1-21.1.4
[✔] twilightdelight-3.0.2
[✔] twilightforest-1.21.1-4.7.3094-universal
[✔] underwater_village-1.0.2-neoforge-1.21.1
[✔] valhelsia_core-neoforge-1.21.1-1.1.5
[✔] VisualWorkbench-v21.1.0-1.21.1-NeoForge
[✔] waterframes-NEOFORGE-mc1.21.1-v2.1.14
[✔] watermedia-2.1.23
[✔] watut-neoforge-1.21.0-1.2.7
[✔] waystones-neoforge-1.21.1-21.1.15
[✔] YungsApi-1.21.1-NeoForge-5.1.4
[✔] YungsBetterDesertTemples-1.21.1-NeoForge-4.1.5
[✔] YungsBetterDungeons-1.21.1-NeoForge-5.1.4
[✔] YungsBetterEndIsland-1.21.1-NeoForge-3.1.2
[✔] YungsBetterJungleTemples-1.21.1-NeoForge-3.1.2
[✔] YungsBetterMineshafts-1.21.1-NeoForge-5.1.1
[✔] YungsBetterNetherFortresses-1.21.1-NeoForge-3.1.4
[✔] YungsBetterOceanMonuments-1.21.1-NeoForge-4.1.2
[✔] YungsBetterStrongholds-1.21.1-NeoForge-5.1.3
[✔] YungsBetterWitchHuts-1.21.1-NeoForge-4.1.1
[✔] YungsBridges-1.21.1-NeoForge-5.1.1
[✔] YungsExtras-1.21.1-NeoForge-5.1.1

Setting the boss spawn cooldown to 1 does not force bosses to spawn, as it does not change the spawn chance at all.

There are way too many factors at play for me to give you individual guidance as to why you aren't seeing invaders spawn; they system does work, but depends on how many available mobs are spawning on the surface. The default chances are being increased in 8.3.0, and there will be an environment variable you can set to further debug spawns in that version as well.

If you would like a more immediate solution, you can datapack the spawn rules to increase the spawn chances https://github.com/Shadows-of-Fire/Apotheosis/blob/1.21/src/generated/resources/data/apotheosis/data_maps/dimension_type/invader_spawn_rules.json

Thanks a lot! I upped the spawn rates and now they're spawning more reliably!