Hardware wise, the GL.Inet GL-MV1000 "Brume" would probably fit the use case, but it currently isn't supported by OpenWrt (so you'd have to do the porting yourself). A supported (and better-) alternative could be the Solid-Run ClearFog Base A1 (SRM6828S00D01GE008B02CE).
Edit: as long as you don't need much more than 400 MBit/s, the FriendlyElec NanoPi R2S could be a cheap and very small alternative as well.
No, I don't have personal experiences with either of the listed devices - but was extrapolating from the information about the used SOCs in there (mvebu can deal with 1 GBit/s WAN speed, the nanopi r2s apparently with around 500 MBit/s - but it's much cheaper). Given that both options raised by the OP didn't have wireless capabilities, I omitted that necessity from the feature set.
gl-inet have their own gui on top of openwrt, which i do not use at all
from that gui, there is a link they call advanced that takes to the standard openwrt luci web interface.
and of course, i just ssh in the the openwrt command line.
imho, i would use the gl-inet support firmware and get great tech support from their forum.
they have a bunch of openwrt experts and very friendly.
Well they are a boring and cheap solution for your use case: Linksys EA4500 , $20. I've purchased several, none run hot. If you have a space consideration, take it out of the case, cut the antennae off & mount safely on bushings in your lan box. I've done this. Until openwrt 20 or 21 come along you could use the Linksys ea3500, smaller form factor, runs cool, $10, but only 64mb memory. Both are hardware overclockable for increased throughput, and reduced latency. EA6350v3 if you want unnecessary but increased horsepower, hard to find good one on ebay $40.