Yes, you need DSC=DSC status ON=ON or OFF=OFF for Windows=Game for faster Alt+Tab
Keep everything the same DSC state, VRR state, resolution state, refresh rates, timings, ToastyX numbers, not just the bitformat same (6bpc, 8bpc, 10bpc) and HDR format same (on/off/colorspace/etc) too.
It's all the same mode change arithmetic. One teensy binary bit different, boom, slow Alt+Tab for FSE.
If you want a different mode for your Windows, and for your FSE game, then you have no choice but to tolerate the slow Alt+Tab behavior.
Often times, the only time Alt+Tab is fast in FSE is an unmodified DisplayPort signal (identical cable timings & protocol Windows->FSE) and it's a lot more work now to convince the drivers that the mode the same.
15 years ago, you only had to worry about a simpler EDID, now you've got a soup of additional display formats and protocols available (VRR, HDR, DSC, etc) and the display almost has to reboot-reinitialize when something changes.