I would be interested to know if anyone has tried the solution described above for Sunrise and the UDM-Pro with the Salt-Box and got it to work? Reports on experiences are very welcome.

I have seen other posts from people saying it works by using the DHCP opinion 60-61 and by configuring a VLAN10

Yes working like a charm

Possible with a provider like Init7 (and used by me). Unfortunately, Swisscom only has GPON, which doesn't work. Swisscom is also intended more for "normal" users, who are also satisfied with the IB4.

Did you tried to sort that out with another provider than Swisscom? Example: iway among others?

That would indeed be interesting in order not to have to buy the XGS-PON fibre router AX7501, since the Swisscom routers are not a real alternative either.

Which transceiver should be used for the XGS-PON? I.e. without a modem, directly from the OTO with fibre to the transceiver in the UDM-Pro? Second question: -According to a Sunrise technician, the MAC address of your private modem is the password for XGS-PON. If the UDM-Pro works via XGS-PON transceiver, which MAC is valid? - The one from the UDM-Pro or does the transceiver also have something to say?

I am not sure if a transciever works here.
There are differences here, GPON (1GBs fibre) or the XGS-PON (10Gbs fibre). I am researching whether I can connect my IPS (Sunrise 10GBs) to an SFP+ port (router ala Dream Machine Pro) with an XGS-PON transciever. The Dream Machine Pro should be able to "translate" something...?
According to various forums, this should work with GPON, but I have never tried it. If anyone has any idea and experience, it would be great to report it here.

I would be very interested in a report on my experience! I am not yet convinced that swisscom will allow this...

This should be possible. However, there may be problems with the iptv.