iMac 27” mid-2010 as Target Display, from 2020 M1 Mac Mini, using Big Sur

I have this set up working OK using the Morphie USB-C to Mini Display Port cable

https://www.apple.com/shop/product/HN8B2ZM/A/mophie-usb-c-cable-with-mini-displayport-connector

Sadly, it takes up one of the precious Thunderbolt ports on the 2020 Mac Mini (there are only two) and suitable USB-c hubs seem to be rare without adding unwanted extras. Consequently, I’ve maxed out by using the other to an external SSD drive, which is much quicker than any of the USB-A ports.

It would be great if instead the iMac Target Display could be driven from the HDMI port that’s on the Mac Mini. Though Belkin do have an HDMI to Mini Display port cable, I haven’t been able to get it to work.

However, every time one of them goes to sleep or I want to restart the Mac Mini, I have to

• disconnect the Morphie USB-C / Thunderbolt cable,
• re-establish the screen working on the iMac again, by gently pressing the power on button on the back and tapping the iMac keyboard (I haven’t worked out the correct or optimum combination of taps and clicks yet, it just seems to work eventually!)
• reconnect the Morphie cable back into the Mac Mini
• re-adjust the brightness again using the iMac keyboard
• all works happily again

Adobe LightRoom and PhotoShop, Adobe RAW, and Capture One 21all work brilliantly well with almost seamless image rendering speeds.

Things that DO NOT seem to be working OK are:

• the F2 button switching on the iMac — which I think would mean I wouldn’t have to keep dis- and re- connecting the cable from the port and must be impacting on their lifespan.
• the DisplayCal colour calibration profile which worked so well on the iMac fails miserably — maybe I’ll be able to re-calibrate when I find he spectro-Munki-Monaco-gizmo amongst all the piles of mess!

Apart from all the displays not being allowed to ‘sleep’, everything appears to be running pretty cool :-)

Does anyone have any ideas about bringing the HDMI port into action with the iMac, and or the reconnection issues?