I wrote below quite a while ago — at least more than a year ago. So some of the info contained herein must be outdated now, if not downright false. It’s just bits of random info — probably not helpful to anybody but me.
2. Client Mode
There were a few resources, but
Routed Client with relayd (Pseudobridge) is the definite one and I should have stuck with this from the get-go.
How To – Relayd shows how to do it with Luci, but missing or not stressing enough a few important points. Install relayd, luci-proto-relay
LAN-side IP has to be static, pretty obvious in hindsight
has to give an IP of a different subnet for relaying
Still not sure how to stop the DHCP server (not client) on the LAN side from Luci, but I made sure I did that with uci.
3. Using External USB Memory Sticks to Increase Storage