I can recommend both Matrix and Discourse forum as excellent open source selfhostable alternatives.

I like matrix, its like the new IRC for almost real time developer chat, I also do this via Discord but I'd prefer an open alternative. If you have experience running a matrix server do you find the data storage an issue? It sounded like if you federate and join the bigger channels you can balloon your storage pretty fast.

	▲	Scaled 2 days ago \| parent [-]
		I mostly host for myself and a couple friends, so data storage hasn't been an issue. I'm sure at some point I may need to prune the attachment store, but I put it on a spinning HDD so capacity isn't an issue. (You can rent a storage VPS and those tend to have oodles of disk space.)