Just pushing this out here, for all the folks that think about using openWRT on one of these old devices:
OpenWRT (version 21.02.0) works, but the performance of the repeater is still abysmal and relative close to original AVM stock firmware (which is also abysmal).
It is adviced to use this repeater only in emergencies and/or when no sufficient funds are available to buy new hardware.
My test results:
I get a maximum speed of ~60-63 Megabit provided by my Internet Service Provider (ISP).
By connecting to a LV-AC05 repeater, which is behind a Fritz!7490 router with my Intel wireless lan 9260 (160MHz) adapter, I measure barely any decrease in performance and am able to reach this speed. See here:
Albeit not a completely accurate measurement, because i do not substitute the LV-AC05 with the Fritz!300E repeater (i am lazy), putting the Fritz!300E between the LV-AC05 and my Intel 9260 adapter, i see a substantial decrease in performance. See here:
I have noticed such a speed decrease also when connecting the Fritz!300E repeater directly behind my Fritz!7490 router, i apologize for not providing these more accurate speed measurements, but i do not want to change my existing repeater configuration.