Connect 2025 Mac mini to 2017 iMac as display (no TDM

You don't.

It's no longer a supported feature in hardware or software.

You'll need to spring for a dedicated monitor for your Mac mini.

That setup is no longer supported by Apple. The old iMac cannot be used as a monitor for any of the newest Macs and Target Display mode is no longer a feature of macOS, having been deprecated years ago. Apple killed target display mode hardware when the first Retina Macs were released in 2014 and the feature died in software with the release of macOS 11 Big Sur.

Please see: Use your iMac as a display with target display mode - Apple Support

Per the link above:

• Only certain older iMac models, introduced between 2009 and mid-2014, could be used as target displays and required either a Thunderbolt or Mini DisplayPort cable.
• The intended target iMac must be running macOS 10.13 High Sierra or earlier; it does not work with macOS 10.14 Mojave or later, or with Boot Camp and Windows.
• Retina display iMacs won’t work as target displays.
• In all cases the source Mac that you're connecting FROM must have been introduced in 2019 or earlier and have macOS 10.15 Catalina or earlier installed.

Please also see: Using an iMac As A Monitor (Thank you, John Galt)

No, you can not use the iMac as the only display for a new Mac mini.

If you have or get a Mac mini, then you need an External Display or HDMI TV to setup and use the Mac mini.

see > Connect a display to Mac mini - Apple Support

and > Set up your Mac mini - Apple Support

Then you can use something like LunaDisplay or DuetDisplay to use the iMac as a secondary display.

To add to what the others have said, Apple released two sets of 21.5" iMacs in 2017.

The iMac (Retina 4K, 21.5-inch, 2017) had a nice 4K Retina screen, and I can see why someone might be willing to put up with the drawbacks of a workaround like Luna Display to reuse it as a second display.

There was also the iMac (21.5-inch, 2017), a price-point model that had a 1920x1080 pixel screen. With the prices of real 24" 1920x1080, 27" 2560x1440, and 27" 3840x2160 monitors being where they are, it just does not make a lot of sense to go to much trouble to use that screen via a workaround.

You're welcome.

👍🏽