I got developer mode enabled on my phone and it allows me to select an alternative for WebView, but there's no options available. Its terribly annoying because a browser is about customising the WWW to your tailor. Not possible with WebView. Adblocker for example doesn't work. Though microG doesn't implement Google Ads.
I think that option only exists to taunt and laugh at you, not to actually change webview.
For adblocking I use pihole in addition to uMatrix on the Firefox Mobile browser (and desktop), it gets rid of 99.99% of ads.
AdAway is another option, if you have a rooted phone and don't have pihole on your network.
There are options that don't require root.. like Blockada and DNS66. Both of which are on f-droid.
I run Pi-Hole in Docker and connect to it via WireGuard. I then use a forwarding DNS server which catches all DNS requests and uses DNS over TLS. Hardware used: EdgeRouter-Lite. Results: slightly higher latency, obviously, but no ads and secure DNS traffic and secure traffic regardless of where my client resides or over which network. I also use LineageOS with microG which doesn't implement Google ads in the first place, and Firefox + uBlock Origin just in case. uMatrix is good, but too much of a hassle to configure IMO.
The list of allowed WebView implementations is baked into the ROM and, by default, only includes the AOSP WebView and various flavors of Chrome.