Pinebook Pro?

[cocozozo]

If I recall correctly, for all new iMacs (after 2017?) to be used as monitors, the USB C had to be Thunderbolt 3 (or at best you could use Thunderbolt 3 to Thunderbolt 2 dongle).

I wouldn't bother with any dongles or trying to chain them, @cocozozo . Getting your iMac to be used as a monitor with non-Thunderbolt devices, will be an expensive and fruitless effort, unfortunately.
A decent secondary monitor would be a much better solution and will work (regardless if you will use for the iMac and/or the Pinebook Pro).

Damn..... seemed too good to be true. Thanks mate.