More spatial streams the better. No, not because you have clients that can match 4x4 in 2.4ghz but because of RF design issues:
- even with 1 or 2 stream clients - more radios on the AP add gain, and 4-stream AP's are always 4-stream, so precoding helps here - RF diversity gain for the extra radio, and coding gain from MIMO
- However, there might be edge cases where an untested design in the lab whiteboard (IP protected) 4x4 radio loses to a robust real world battle tested 2x2 RF design.
RAM doesn't matter if you aren't using openwrt in "dumb ap mode" aka ghetto mode, which you stated above.
There are poor design cases with some ath10k radios with RAM problems but you qualify your statement as 2.4ghz only.
Enterprise "true" AP mode have linux bypassing kernel acceleration or "hardware acceleration" if their marketing teams find something to qualify it as such. An example would be the NSS sub processors (7200k pps in 64k! packets) in the ath 10k 999x and 998x series. Normal linux bypassing would be 640k pps on 1500k packet size, so probably around 100-150k pps on 64k packet size.