Free and Open Source Software

I'm looking for a simple alternative to Splitwise to self host. Currently my partner and I use it to keep track of monthly expenses to ensure there is roughly even spending between us. Doesn't have to be complex, in fact the simpler the better as my partner prefers simple, ready to use tech. It would only need to be accessed locally as we update expenses once or twice a month, and would prefer FOSS to have control over our data.

I thought of having a Spreadsheet that is synced using syncthing between my computer and her phone. This seems simple on my end, but I worry that it would be more complicated for my partner as they primarily would use this on their iphone. Currently on splitwise you can add expenses with only a couple button presses.

Thanks in advance.