This is awesome! I've also been doing development on an old Mac 512k (upgraded to 2MB). It's pretty wild and super fun.
Until recently I was using Think C, which was great, but recently I've switched to Think Pascal. It's true what you read on the old forums, Think Pascal had more love put into it. The editor is better, the errors more helpful, and little things like remembering which files you had open in a project make a big difference.
Shout out to the Mac 512k Blog[0], which has some great content as well.
(Edit) Also for anyone else looking for helpful online resources (beyond tutorial books and Inside Macintosh, which are essential), preserve.mactech.com has tons of great old articles.