How to configure HiDPI resolution for an AirPlay display?

[Cimlah]

Hello,
I got a new MacBook recently, but I wanted to make a use of my older iMac, so I use it as a second display for my MacBook. I use Apple's built-in Screen Mirroring feature for this. When I connect over WiFi, HiDPI profiles are available.
The problem is, when I try to do the same thing over Ethernet. My iMac is also connected to a network switch over a 1Gb/s Ethernet and I'd really like to use it instead of WiFi. However, when I turn off WiFi on iMac and use Ethernet only, the HiDPI profile is available only for 720p. WHY?
Another problem I have, is that half of the options in settings are not available for the AirPlay display, such as flexible scaling and custom resolutions. I tried to create a virtual display and stream it to my iMac in order to achieve HiDPI (and it is accessible), but for some reason it just doesn't look as sharp.
My goal is to get HiDPI for at least 1920x1080 resolution on my screen-shared iMac over Ethernet.

Perhaps there are some hardware factors to it, so here's a list of my gear:

• 2023 MacBook Pro M3 (base)
• 2019 iMac (base)
• 1 Gb/s TP-Link network switch (router and iMac are connected to it over a cat 5e cable)
• 1 Gb/s TP-Link wireless router (MacBook is connected to it over WiFi)

Over WiFi:

Over Ethernet:

[damnventures]

Doing the same right now with my 2017 iMac. Some things I discovered:

• Native HiDPI for 1920x1080 working using thunderbolt 4 cable from iMac to MacBook.
• "AirPlay display" in the BetterDisplay app is so limited, no scaling for sure.

Trying to find a way to turn HiDPI for the 2560x1440 but no luck so far.

[Cimlah]

Could you expand on it? Are your MacBook and iMac connected with Thunderbolt cable directly? Do you use network over thunderbolt to stream?
Simply saying: How exactly did you manage to do it?

[waydabber]

I think the better method is to create a virtual screen and use screen sharing from the 2017 iMac to the MBP, selecting the virtual screen as shared screen. This should give flexible resolution (you can create a 2560x1440 HiDPI desktop this way). Trying to push the screen as AirPlay is a misery as this feature was aimed for TVs.

[Cimlah]

Yes, indeed. Creating a virtual display enables me to use HiDPI resolution. BUT, as I mentioned in original post, for some reason this HiDPI definitely does not look like HiDPI. I guess there could be some network factor to it.