I have seen this solution, which apparently leverages something called v2ray. However since I never heard of that software until today (and I can find very little information on the Internet about it), I find it a bit sketchy (no offense to the author) and therefore am very disinclined to install it on my new Beryl router.
Surely there must be some easier / more straightforward way to simply specify an http proxy as an upstream Internet connection (in repeater mode), no?
Currently I am doing the same as this person, which works well enough from Android (requiring only a single, simple setting when connecting to the Wi-Fi hotspot). However this becomes much more problematic when connecting from Linux, as proxy settings there seem to be scattered all over in various places.
I would much rather have the router handle all of that complexity, such that I could leave all my end devices alone. In fact I've recently purchased a new Beryl router (upgrading from my old AR300), but so far haven't been able to get this working in the desired fashion. 
EDIT:
Eventually I would also like to create a "site to site" Wireguard tunnel between my Beryl travel router and home network, such that all devices can reach one another just as if they were on the same LAN. Perhaps that is best left for another thread, however I thought I would mention it in case it might be relevant to the current discussion.
EDIT 2:
I would in fact be quite happy to pay someone to help me sort this all out, as it is just out of my reach and I don't really have time to faff about with it and need it to all just work.