1) Moving the code to the RP2040 from the Raspberry Pi foundation. This would make it easy to add a WiFi interface for remote control and display. The RP2040 is not very expensive, $4 without and $6 with WiFi.
For this sort of thing, I'd prefer a wired approach, setting up a reliable Wi-Fi is a pain.?
I did wonder about Arduino or Pi for the local end in a remote setup, but the PIC controllers are cheap...

2) Double up on a couple of the relays to approx. double break-down voltage on the higher impedances.
I'll consider that if I add an extra inductor.

3) Build a 100 watt attenuator into the circuit. When tuning, the signal would pass through the attenuator with about 1 watt radiated when 100 watts is input. This makes sure the radio never sees high SWR and that the relays are now switching under high load/voltage. A circuit would compare the impedance on the input and output of the attenuator to do the tuning. The SWR bridge would be used to check the tuning when the attenuator is out of circuit. For an example of this, see the Icom AH-4. See ?. I believe the service manual is available on the groups.io site.
I've looked at several remote tuners (the SG-237 service manual has the full schematic which is quite interesting).

4) Maybe separate the tuner onto two PCBs. The one PCB would be the tuner and handle all the RF stuff with space left open for some of the RF enhancements. The second PCB would contain the logic. That would allow changes to be made without having to throw populated boards away.
I'm looking at an add-on board for the current design, it will have the frequency measurement stuff on it, and maybe an I2C EEPROM.
The route I'm looking at is a switching and display board, which can be in one box or remote.

5) The best place for a tuner is at the antenna feed point. So anything to support that is good.
Yes - This is my experience, either ladder line to a local tuner, or remote tuner + coax.
I'm thinking about using cat5e/6 for the remote link cable, this should be good enough for power+control+gnd, so just one control + one RF.