What about one of the antaresia species like Spotted Pythons, Childrens’ Pythons and Stimson’s pythons? They would eventually outgrow the size of enclosure you mentioned though, but that gives you some time to get something a bit bigger.
I considered them as an alternative to ball pythons in my research and they are partly arboreal. Much easier care requirements than GTPs, Carpets, Amazon Basins etc and they’re easily found reasonably priced here on MM (Childrens have their own subcategory, the others you’ll find under “other (pythonidae)”) Based on my research they’re usually really good eaters once they’re started as well.
GTPs are so beautiful but I think it’s generally recommended to buy US captive bred individuals if possible, which is gonna be well over that $500 mark you mentioned upthread.
Is the size of enclosure due to budget or space constraints? Because I think that changes the answer a lot…