Thanks everyone for your inputs.
I ended up buying the DIR-2660. I'll first test it with the stock firmare to check that everything works fine and then I'll try to flash OpenWrt.
The HW version is A1 with firmare 1.03 out of the factory. Should I avoid upgrading the firmare while testing it or, as I suppose, it doesn't make any difference for the future switch to OpenWrt?
As I understand, to flash it, I'll have to use the images I can find here:
https://downloads.openwrt.org/releases/21.02.0-rc1/targets/ramips/mt7621/
using the D-Link Recovery GUI method:
Anything I should be wary of while trying or, as stated, is it actually impossible to brick the router using this method?
Hopefully it works fine as a few months ago people seemed to have problems flashing this router: