I have this issue where the custom mobs doesn't spawn at all, they just appear very rarely.
what I see most is the ghost mob, that mob appears everywhere! except the others.
Grimoire of Gaia is different, all the mobs from that mob spawns naturally in the overworld, everywhere. it is really nice.

any chances this could be fixed?

I will assume the answer is yes, but have you tried tweaking the config files to make sure? make all the chances of spawn 100% and then check if they still don't spawn

Probably it's intended. At least since 1.16.5 most mobs have been rare in spawning. I think it's a part of balance - otherwise those powerful items are too easy to get. Gaia mobs can frequently spawn because most of them don't drop rare items, or the drop probabilities are very low.
If the mobs don't spawn at all, maybe you need to check the biome and height. For this, there's a wiki but in Japanese: https://forum.civa.jp/viewtopic.php?f=3&t=750