Xiaomi - Xiaomi WR30U

Running 4 of these in different locations for more than a year now without any glitches. A very capable model with Wifi6, 4 gigabit ports, 256RAM and 2 ARM cores. But beware of different revisions -- the most recent one (RD03v2) is not supported, since it's Qualcomm hardware instead of MediaTek

Based on your requirements, Xiaomi AX3000T is your best choice and it's only $32 in my country.

I am using Mi Router 4 and Mi WR30U myself. Both are cheap but the WR30U has worse documentation for being less popular.  If you are only looking for a small upgrade without going extra, Mi Router 4 (not 4a) has 128MB flash, mainline OpenWRT support and decent documentation. It can be found for a little as ~$7USD second hand so it will fit in your budget even if you spend all of it importing 

I got Xiaomi AX3000T for 36€ on BangGood as first time customer. I would say pretty good deal. I don’t know if it’s old, but price is gold.

Xiaomi AX3000T - has 5GHz 160Hz which will be more than enough for your speed requirements. Great processor, ample ROM space and RAM.

It depends, Best is **x86** (mini pc or specialy designed x86 platfrom for routers) since you will not suffer any drops thanks to powerful cpu as example **intel n100** it will overpower any other platforms For arm routers look for filogic processor like Gl.inet flint 2(**filogic 830**, 1 gb flash) For cheaper options check official supportted devices on their page look for **filogic 820,810** both are perfect options as well a bit less powerful For example multiple xiaomi routers (but be caureful since they have multiple revisions not all supports openwrt) simmilar brands is Linksys, asus, tplink, cudy, etc For budget options go for **mips** platform, but it will suffer significatly when it comes to cpu performance althout offering low power consumption TLDR: I am using xiaomi ax3000t for now, it comes with filogic 820 , offers good speed and quite stable, only downside if you want sqm to fight bufferbloat, you will suffer drops to 350-450mbps, without it you can achive 650mbps thanks to WED support But again latest revision of this model does not support this router at all, so it took time to find another revision

I get gigabit speeds or very close with a cheap wifi6 2x2 MIMO router from Xiaomi on 5GHz and flashed with openwrt. It cost me around 35€ on AliExpress. If close to it, I get around gigabit speeds (800 to 900 Mbps). Going further away it can drop to around 500 Mbps.