Upgrading to Eero WiFi Pro 6E, and moving the gateway closer to my desktop system made a dramatic difference.
First, relying on some some older ethernet cables (Cat 5) already in place, I was able to put the gateway next to the downstairs desktop (Apple Mini M1) system and directly cable the Eero to the fiber modem in the garage, and connect the Mini directly to the Eero.
So now the desktop system is getting 980 Mbps both ways. And wifi downstairs is getting 180-250 Mbps all over the downstairs (as measured by my cellphone). I put the secondary Eero in the same upstairs room that has my laptop and TV. The wifi now upstairs is getting +/- 200 Mbps. I've also cabled my laptop to the Eero and it's getting +/- 400 Mbps consistently.
I consider this a success, since the speeds I'm getting now are at times 10x what I was getting before in every configuration. Even with VPN, I'm getting 350+ Mbps with the ethernetted laptop. And I know that if I need to upload any large files, I can do that from the desktop at near Gigabit speed.
Thanks for all the suggestions.