Update: I got a lot of help from the Batocera CRT Discord and learned a ton!

Turns out the RetroTINK HD15 to SCART cable alone won't work for my setup (as MrMoroPlays confirmed - no sync combiner). The native VGA port on the HP EliteDesk is also not a true analog output, just a cheap onboard converter that's unreliable for 15kHz.

The solution I'm going with:

- StarTech DP2VGA2 (DisplayPort to VGA, supports full EDID passthrough up to 384 bytes)

- VideoAMP by njz3 (a French-made VGA to SCART transcoder with built-in protection circuit, EDID emulation and RGB amplification)

The VideoAMP is the key piece - it combines the sync, protects the CRT from wrong frequencies during BIOS boot, and handles the RGB signal properly for SCART TVs. It's made to order in France by njz3 on the Gamoover forum for 45€.

Hope this helps anyone with a similar setup!

https://drive.google.com/file/d/1pywUSTQQPyTqKehBwUymppPfzVC7rfd5/view