This "feature" bit me really hard this summer. You can read my 2 month struggle here: Issues with multiple networks / VLANs with a router and a dumb AP - #20 by paraskevas
tl;dr: I have an ASUS RT-AC58U configured as a dumb AP that mirrors packets back and I suspect its due to how its ports work. The two issues I faced was DHCP working erratically (if at all for some devices) and main router's syslog getting flooded. Performance was also really degraded. My solution was to change the LAN cable coming from the main router from WAN to LAN1. This fixed everything (and my sleeping schedule). I'm still not using its WAN port and I'm going to try @NoTengoBattery firmware when I have the time
Regarding this topic, since I've tried DSA on another router and since (I believe) this is the way forward I find Option 3 the best. A lot of extremely popular routers broke when upgraded to 21.02, so I don't think this is an issue with this platform. Its less popular and this is interface issue is known for quite a while. Also this kind of config breakage is supported by OpenWrt internally, so no work to be done there. Users will be prompted before flashing and chose to either upgrade or stay at the old version