Using 2017 imac 21'' as monitor in 2024

Unfortunately not. The last iMac that supported that was the Mid 2014 model. There are a few more OS restrictions. The operating Mac must be from 2019 or earlier and have macOS Catalina or earlier installed.

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

there are third party ways though

Use iMac as monitor: 5 Solutions - 9to5Mac

but judging how much more power 2017 imac use vs. any 21" monitor it seems a bit wasteful

Purchase a real external 4K display in 27 or 32 inches that meets your budget and workflow needs. Additionally, use a quality 4K HDMI 2.1 cable to connect them such as this Belkin cable.