If it helps, think of it like running. If you’re running your favorite, well trodden route, you get a feel of your pace & effort level. If you’re running on the track, you get the same and (should) start to be consistent with each 400, 800, etc without looking at your watch, or needing pace alerts, throughout.
Doing time-based, not rest-based, intervals will work. And you can program these workouts into your Garmin. For example, do 10x50 on a specific interval, say 1:00 or 1:30 (or :55… etc). Whatever gives you the rest for the target (speed with low recovery, all out with high recovery)… perhaps start with “strong, maintainable effort” so aim for 15s rest. If you know that pace, and this conversation is based on not really knowing it, program the workout into your Garmin as your target plus 15s (or 10s, 20-30s is a bit high for this). You’ll hit start (ignoring/simplifying programming warm up and other parts of the workout etc) and hit lap when you finish the interval (the 50, in this case), and it’ll automatically start the countdown for your send off, at which point you just go (no button press). And repeat. Doing a set interval will help you get your pace. Where you are now, rest-based intervals will not. Having the workout on the watch is immensely helpful IMO. My Fenix5x will vibrate halfway through the last lap to tell me to stop at the wall, which is super helpful on 400’s and 500’s, even 200’s in my case. While I could glance at the watch coming off the flip turn, it is easier not to and to get the vibrate to confirm my count (or more likely to inform me when I completely spaced on the count, which is not uncommon as I am not swimming with a team or anyone else though i am usually in a shared lane with people doing something very different and almost always they are slower).
I hope that helps.