If it ain’t broke, don’t fix it. I think it’s better hooking up Element Call to your current setup, and remove Element Web if you can BYO client.

For a more lightweight alternative, I personally find continuwuity to be reasonably stable for the specs you mentioned. It does admin tasks in an #admins room, use an embedded database, and has no client UI so less containers needed. So continuwuity + EC should be able to run under the constraints you mentioned

The lightest would still be any XMPP server, though its functionality does differ from Matrix overall