2017 iMac as display for any new MacBook/Mac Studio?

That Mac does not support Target Display Mode – and no current Mac supports using ANY Mac as a Target Display. It is also too old to act as an AirPlay Receiver for AirPlay to Mac.

See the bad news here: Use your iMac as a display with target display mode - Apple Support

See the bad news hers: Continuity features and requirements for Apple devices - Apple Support

There might be third-party workarounds that would let you use it as a second display. (You'd still need at least one real one, such as the MBP's built-in screen.). But your tag line indicates that you have a 21.5" iMac. That could be one with a 1920x1080 screen or one with a Retina 4K screen. If it's the 1920x1080 one, it's definitely not worth the expense and hassle of trying to reuse it, and even if it is the 4K one, the drawbacks might make that marginal.

You should plan on purchasing a real display.

There are third party software/hardware option. I use Luna Display. There is a dongle for the primary computer (Macbook) and software for the secondary computer. For me it works well when both devices are connected to the same Ethernet network. It works on Wi-fi as well but I wouldn't recommend it. I would agree with Servant of Cats plan on getting a real display would be your best bet in the long run.

Hope this helps.

Paul

Third-party workarounds are virtually all laggy to some extent. Example: Luna posts this on their support page:

The red emphasis is theirs, not mine. To me, that says Luna cannot recommend itself as a primary display for a Mac like a Mini of Mac Studio requiring a separate, when even ads on web sites need full-motion video. It could useful on a computer with a built-in display. There, an iMac would serve as a "parking lot" for static windows and palettes.

However, an iMac would use far more energy than a free-standing external monitor and may not allow full external display options.

Well a 2017 is obsolete meaning Apple has no updates for it and it cannot run current version of Mac OS. Apple has also classified it as vintage meaning parts are no longer available. However if you want to keep it you can of course and use it for non essential tasks. You can also recycle it using one of Apple's free recycling programs or donate it too.

That iMac can run Ventura – still one of the "most recent three" – and the latest production version of Safari. When Apple releases macOS 26 ("Tahoe") this fall,

• Ventura will no longer be one of the "most recent three".
• Ventura will no longer get the latest Safari (although it might get some bug and security fixes to Safari).

Safari Release Notes | Apple Developer Documentation

The ship has already sailed on the latest versions of Pages, Numbers, Keynote, and GarageBand – which all require some flavor of Sonoma, or higher.