Still on WHMCS, but did test Upmind, didn't like it - found really basic bugs, support was poor, don't like the idea of SaaS where they might go down or getting locked in on a newish service when they could raise prices leaving little recourse to migrate away, etcetcetc. At least with the big name systems you know it's easy-ish to migrate between them as needed.
There's some cool open-source billing panels that look interesting... I haven't tested, but you might want to check them out:
https://fossbilling.org/
https://paymenter.org/
There's a couple more I had been checking out the last few months, can't remember what they're named now. Open sourrrrrrrrce š If your business needs are pretty simple, one of the open source systems could work!
