Last I checked, Fathom’s open source product hadn’t been updated for a couple of years. So, I switched to Plausible which is more reasonably updated.
GA4 migration seems not aimed at general users, so I'm looking at alternatives. Ideally could import my data.
Plausible is much simpler than UA, all the reporting is on one page and it has much more limited config options. It also can import your UA data. If you're looking for a simplified GA alternative it's a very good choice. (Full disclosure, I recently published an article on their blog about UA vs GA4: https://plausible.io/blog/ua-vs-ga4)
I'll have a look, seems promising.
Good luck!