Presnt the user with an indicator to help them locate the coordinates without relying on f3, or mods like minihud.

For example, JustMap's waypoint renderer:
https://github.com/Bulldog83/JustMap/blob/1.16.2-ulimited/src/main/java/ru/bulldog/justmap/client/render/WaypointRenderer.java#L85-L220

This would be very helpful for helping users return to previously marked coordinates.

This has been in the back of my mind to do, however I haven't figured out the best way to do it. I really wanted to avoid the idea of waypoints per se, as they create a lot of clutter onscreen, but there definitely needs to be an indicator of at least what direction to go.
Possible solution would be to just render a compass above the HUD for the overlay text, or utilize the compass item in some way.

A compass works, but how about a beacon beam at the desired location?

It'd be visually in the style of minecraft, and would only apply to the selected waypoint

and would only apply to the selected waypoint

yes, it definitely should only apply to a selected since otherwise it's just an minimap [mod] without a minimap and at that point why even bother