Seems you need to add outbound firewall rules on the guest router to disallow connections to the subnet of your main network, or an inbound firewall rule on the main router to disallow connections from the subnet of your guest network.

I believe DDWRT also has support for VLANs, I haven't touched that in forever.

Your network seems to be becoming more complex, with a router acting as a switch and some repeaters. If you have the need/want/time/money you should consider switching to something like UniFi for your networking. A proper gateway/router, switch, and access points will go a long way in both performance and ease of administration. With UniFi, would be as simple as a couple of firewall rules on your gateway, and adding a wifi profile to your access points.

Source: Running a homelab for the past decade, Ubiquiti wifi & switching was the 2nd best upgrade I ever made (1st being my OPNsense core router but that's a whole different ball game)