In short, the custom PCB and housing are done and ordered!

Some details on the PCB

For the PCB, I continue using the Raspberry Pico W.  I decided not to include a external USB-power supply, but include it on the PCB, as mains already goes over the PCB anyway to switch power to the amplifier.

For the parts, I limited myself to those available at a local shop (Segor in Berlin), so I could easily pick it up. So, I chose a Myrra 47152 as an integrated AC/DC converter for the 5V supply.

For the relay K1, a Schrack RT424005. Initially, I chose a Hongfa HF115-F-Q, until I saw that that one didn't have reinforced isolation between the coil contacts and the power contacts, which could lead to safety issues. As the Schrack was also available (and even had a Kicad symbol+footprint) it was a better choice.

All the components are through hole for easy soldering. I reserved the space for the Pico anyway, have those in my personal stock and they are just easy to work with. Volume is not a problem.

Furthermore, I've added a pair 3.5mm connectors, not for audio IO, but for connecting a button or analog sensor. Don't know if I'll use them, but nice to have them easily available. They don't have any function in the software yet.

Pinning as follows:

• Tip: GPIO28_ADC2 / GPIO26_ADC0
• Ring: GPIO21 / GPIO20 (switchable 3.3V supply)
• Sleeve: GND

That way, I can use momentary buttons (with the internal Pico pull-ups) on Tip/Sleeve, or I can wire up an analog sensor like a potmeter and read it back if I want.

Other things:

Mains comes in on J5: the same connector the Marantz uses, so I'll reuse it's cable.
Mains goes out on J6/J7, 2 faston terminals. I'll hardwire these to the Marantz mains inputs (post-relay) as shown before in the log "Mains switching".
The RC-5 header connects to a RCA-connector (feedthrough variant) I had laying around. That in turn connects to the Remote IN on the Marantz.
The PowerSW header connects to internal micro switches from the log "New power switch and repurposing the button".

Now it is waiting on the bare PCB, so I can solder everything and test it completely.

I'll publish the Kicad files (schematic + layout) later.