We are running our infill site on 4G at the moment while the normal ADSL connection has an intermittent fault - I know from experience it takes Openreach months to solve these sorts of problems. Like you we run it over a VPN (Tailscale - free)
It runs really solid - maybe the odd dropout of few seconds every few days - but they tend to be in the early hours of the morning when no one is listening anyway.
Big question in my mind is how good is your 4G signal? We are using a Microtik outdoor 4G router to try and maximise signal. The way 4G (and 5G) works, the better you can make the radio conditions of the cellular link the more reliable signal you will get (ie not only peak throughput, but enhanced reliability). This is because the scheduler in the base station recognises how good your link is and thus schedules you more often because you are less "taxing" than users in poorer radio environments - you essentially get an "unfair advantage" over everyone else, even if the cell site is congested! Ideally you want to get 5 bar +++ coverage so that you can use the highest possible modulation scheme on the cellular link (that said we only have 3-4 bars!). It also helps if your 4G modem equipment is a "high category" which means it has the higher modulation schemes and tricks to improve throughputs. Cat 3 is the lowest you can get, Cat 6 is ancient, something like Cat 12 or above is where to aim. If you get a 5G device it is likely to have higher 4G category by default. I'm waiting for our ZTE MC7010 to arrive which looks like it should be about the best you can possibly buy!
https://www.amazon.co.uk/Outdoor-Router ... S22GF?th=1