@sky-runner you don’t need FIT to do that, despite all the posts above saying it…GPX will suffice, it’s exactly what I’ve been doing before returning to Suunto this year.
For previous few years I’ve been using plotaroute, adding my own waypoints (technically they’re routepoints) to mark turns/aid stations/whatever, exporting as GPX and successfully using those files on Apple using the Workoutdoors app, or on Garmin (with the oddity that I had to dump the files into the watch file system via a browser, sharing them via Garmin Connecf would strip my waypoints/routepoints).
It became my much preferred method of watch nav, Garmins automated TBD is just stupid, with tons of spurious alerts, and Apple Workoutdoors app simply didn’t have it, until recently.
Creating manual turn points / cue points, embedding them in a GPX, is the way that I want to work most of the time…I want to study the route and ensure I understand it, adding alerts when I expect to need them. The automated TBD is just a nice to have really.
I think the limitation you’re referring to is for POIs, which technically sit on the map, rather than on the route, so have no concept of being “at X distance along the route”, they’re just “oh, you’re near this”.