I mostly note down my transactions from my phone and https://moneymanagerapp.com/ has been working great for me.
It does let you start a webserver from your phone and use it on your desktop if you really have to, but the workflow is not ideal. It also does would not work if you need to track expenses of multiple people or want to automatically import expenses from your bank. It does allow you to export your data though. Backup and restore can be done via Google Drive.
Not OSS, and also paid(one time), but was totally worth it for me.
The simplest way that I find to get an webapp running is by using [parcel](https://github.com/parcel-bundler/parcel) for bundling the frontend and using a python [flask](http://flask.pocoo.org/) server for backend. I don't think it could get any simpler than this. Now if I have a database, I usually go with mongodb along with pymongo.