Hey @SoloMSP and welcome! Appreciate your kind words.
We mostly support T&M billing via tickets with the time tracking - set the client hourly rate, make the ticket billable and you can invoice it. You can do the per user / per month with recurring invoices (but you'll need to adjust them before they go out for any changes like new users).
Prepaid hours / Credits has come up before and is something we might add eventually (but like you say it's a fair amount of work to add properly - dealing with money is always hard).
We are looking to enhance Products - currently it just auto-fills the fields in invoices, but we probably want the link to a product in a draft invoice / recurring invoices to be "live" so that any changes to the product are updated everywhere.
There has also been a more recent suggestion of making assets billable, which could also carry over to licenses & portal users for automated invoicing.
Love the ideas, even if they're something we need to think over, get further community feedback or approach in a different way.