I use aTimeLogger app to track my time then I email myself a CSV report from the app and use Google Sheets to create the invoice using a couple simple formulas and an invoice template I made.

I paste the data from the email into a new copy of the invoice template and it mostly does everything I need. Then I download it as a PDF and send it.

I invoice two to four companies per month. Takes me less than 1/2 hour usually.

But I came here to find out if there was a nice self-hosted alternative.