Correct. The Beryl AX has the same limitation. A switch takes care of that.
A 4K stream on Netflix is ~15 Mbps/client. A commercial VPN provider (eg: Mullvad, Proton) is typically cap'd to 1 Gbps bi-directional. A private WG-based VPN (eg: so you can remote back in while on holiday) is limited the the upload rate of your ISP. Make of that what you will.
Don't beat yourself up. We all started somewhere & your expectations, goals now outstrip the 'convenience' provided by 'appliance vendors' & their associated abstractions (read: limitations). With your GL devices you now have the option to 'pop the hood'.
"Stuck!" LOL!
I've said it once & I'll say it again: OWRT is a helluva gateway drug.
I'd suggest flashing pure OWRT on second Beryl AX (critical note: only the the 'sysupgrade' tagged imaged). That way you can see, test the difference between GL's customization & experiment with 'unfettered' OWRT. I say that because I run GL firmware when travelling with my Slate AX travel router but flash back to pure OWRT when I'm stationary/at home.
Unless the Google devices can be flipped into a simple AP mode or the client device is already on Network A, usually no. Any traffic from Network B is going be seen as one source based on the MAC of the Google device. One way to theoretically get around that could be to just put every client device on a WG VPN & then use those WG IPs for GL GUI's PBR ('policy based routing') but even if able to be done that's going to considerably impact all client's bandwidth performance (eg: Burme v2: 2.5 Gbps v 355 Mbps). So technically yes but.... you're gonna feel some considerable pain. I wouldn't bother doing it unless you just care about getting < dings > to you phone(s) rather than heavy surfing/streaming.
EDIT: based on your diagram all you'd have to do is drop the 'Google Wi-Fi router' behind the switch provided the overall cable runs aren't an issue:
One thing I might suggest as an experiment is to take your second Beryl AX & flip it into AP mode, downstream from the Burme v2. Flip its TX/RX power output to max & see how its range, quality compares to the Google set up.
To go a step further it's not impossible to build your own mesh network on pure OWRT + approp. harware/antennas but that's all dependent on environmental conditions like the building's construction, overall footage, etc. Then you can sell the Google gear on a second hand market to who just want that 'appliance' approach to networking.