What's the best practice for using UTMs for search ads?
You should be using auto-tagging with Google Ads, not UTM's.
Auto tagging is enabled by default and will automatically give you all the information you need to be linked between Google Analytics and Google Ads. This requires pretty much 0 effort on your part and significantly reduces the likelihood of linking/attribution/tagging issues.
The other option is you could use UTM's, which requires you to add them and makes it significantly more likely you run into some linking issue. You still use UTM's just not for Google Paid Media. You simply can't beat the accuracy of Google doing the tagging for you.
It may occur that you get "not set" for your keywords in the attribution model comparison tool in Google Analytics. You can then set up a tracking template at the account level.
Here's a tutorial: https://www.youtube.com/watch?v=o1E_MzoOXr0