The Xiaomi Mi Router 3g was, in my opinion the best affordable router that ticked all these boxes. I own 2.
Unfortunately you can't get them now, Xiaomi stopped producing them and its successor (Mi 4a, often rebranded as 3g v2) is not as good and you have either to rely on a software vulnerability to flash it or have to open it up since it doesn't have USB.
I mentioned that in the original message. It's no longer available.
The ones you can find to buy right now are rebranded Mi Router 4a's (as 3g v2). They have half the RAM, flashing them is not straightforward and according to the wiki they have some major connectivity issues.
Maybe state a little clearer that you mean the Mi A4 Gigabit and not the non-Gigabit, which just has fast-Ethernet... I'm running 2 as simple APs with fast Roaming and for that use case they're great. You don't even need luci then, so RAM and FLASH isn't a concern for me. That's just what you get when you pay less then 30 €...
But you're right installing openwrt on those (if you do use the one in the wiki, it's most stable) is not too straight forward...
If you are in EU the linksys EA6350 is on sale on amazon.co.uk for 29.99GBP. If you don't plan to use ipv6 which is currenlty borken, it is a great option. Last time ordered it was the v3 one.