"self hosted budget management app". Can't you just install this type of app to your phone or pc? What's the purpose here, will you host it and access it from a browser? Or do you only want to backup its data to your server?

I don't want some third party having access to all of my transaction history and knowing what I spend and where.

I hope I don't sound stupid please enlighten me.

Your question isn't stupid. There is an important decision you need to make on "is the juice worth the squeeze." While you can selfhost a lot of stuff sometimes there's better reasons not to. Email is primary example that gets brought up a lot. Sure you CAN self host it, but for a lot of people on this sub it's not worth the effort required to do so.

Each person has to make that decision for each of the things they choose to self host. Budget apps are no different.