Currently me and my GF have our finances organized in a Google sheet file (hosted on Google drive), being that file integrated with a Google form.
What we do is having on our cellphones a shortcut to the form, where we input all our expenses, they are directly and automatically registered in the sheet, and on another tab we've built some sort of dashboard based on all the values the form registers.
So given this context, is there any option or group of options that are open source, and that achieve this same purpose / scenario?
If possible everything acessible on a cloud or at least onlinez so we don't lose this flexibility and accessibility on our cellphones.
I use Grist for this purpose. Check out this template, this may be just what you want and using widgets it is quite easy to create a form to append to the expenses database (just like here). Grist works really nice on mobile too and is also pretty easy to self-host if you need an extra degree of privacy, but you can use the official instance as well.
If you want I can send you my Grist template that does pretty much all things you want.
Also keep it mind that, while Grist looks like a spreadsheet, there are some key differences between it and a typical spreadsheet (see here). You may need to spend some time with the documentation (which is really good btw, and they have video tutorials as well), but in my opinion it's really worth it.