The 80X/AX3000 is Qualcomm, there's currently no support for those in openwrt.
For the 70X you (need to ?) change boot loader to breed, then flash the image of the dir-853-r1.
Once booted, manually export the radio fw by running cat /dev/mtd2 > /lib/firmware/mediatek/mt7915_eeprom_dbdc.bin on every boot, and add the kmod-mt7915e package.
I'll take a look. I found this sale on amazon https://www.amazon.es/-/pt/dp/B07QYGCGS7/ and it is supported but reading the forum there is a problem for futures versions because of kernel size so I don't know if it is a good bet or not neither how it compares with the one you linked. Do you think MR70X will have better support in the future than MR8300?
My ISP connection is 500/100 so if I can get something around 300-500mbps using WiFi it is ok (i could get that speeds with isp router so I think it is ok for a AC2200). Even to use the local NAS these speeds are ok for me using WiFi and I don't think I'll need something better on a few years.
These are my ISP router wifi features (looks like a ac2350).
My main concern is to choose an AP that is most likely to have openwrt in the following years
I can't found any around 60eur, are you talking about this one: TP-Link AC2600 RE650 Gigabit Range Extender - RE650 ? I like the small factor as I don't need the extra ethernet ports. BTW on a dumb ap can I still use the other ethernet ports as a "dumb switch"?