@johnpoz Sorry - should have been more clear (and I'm still learning how/where pfSense falls into place here, in terms of its initial configuration). I was thinking about more 'foundational" features, such as as a DHCP server, DNS, toughening up security with some simple rules, etc, etc.
But your last comment clarifies things, I think. It seems I can simply drop pfSense between my modem and ASUS router and use double NAT. The default configuration you describe sounds exactly as I'd want (and allow my existing devices to continue "blissfully unaware").
The only thing I'd need initially is OpenVPN, since I regularly remote into desktops from coffee shops over RDP (and don't plan on forwarding ports), etc. With VPN installed and configured, I'd be pretty much at parity with what I have now and can then start walking down the path of leveraging pfSense to the full, particularly as I implement IOT isolation across a couple of subnets.
Thank you again, @johnpoz .