Odoo CE (community edition) is open source and has built in stripe integration (along with a few other payment providers) that will invoice on time spent against projects/tasks.

If you’re in a region that has l10n for accounting (via the OCA), you’d be able to handle all your tax compliance also. Only caveat to Odoo CE is you’d need to self host, which in itself is not that difficult.

odoo.com github.com/odoo/odoo github.com/oca