Proxy Relay section in your relay settings. It will still connect with a bunch of relays, but most of the data will come from those relays.
Just make sure the aggregators are good and support all event kinds amethyst supports, otherwise you might miss some stuff.
Aggr.nostr.land works.
WoT relay also works great for this purpose.
It does cut down a lot, but there should still be the option to only connect to those proxies. I'm still connected to between 30-50 relays. It's a good enough balance for me. But I'll keep the bounty open for a stricter option.
