I use Xerca Mod on a server that you can get other potion effects like Haste in but I can't seem to combine these, technically, vanilla haste potions into the Ender Flask which is very disappointing. It'd be awesome to see the Ender Flask at least work with all minecraft:potion items. I can understand not being able to do any modded potion item like what Reliquary adds.

That being said. I love this mod's concepts especially the Ender Flask and Potion Launcher items. They're both really creative and fun to use.

I looked into this and from what I see, this won't be doable for the time being. Potions with effects that you can't normally get are usually just some potion item with no effects (like water bottle) in disguise, with additional NBT tags that add potion effects like suspicious stew. Adding them to Ender Flask would require me to look into potion items' custom effect NBTs (which can be multiple on the same item) and put those as NBT into the Flask/Potion Launcher item, which will end up quite bug-prone. But I may look into this in the future when I have more time.