Change The Activity Type Name on a Workout

I don't think I fully understand what you mean. All the same, the answer is most likely no. Performing a custom workout on your Garmin wearable device (which, I'll assume you've already ascertained, supports the feature) does not make the resulting recorded activity any different from any other recorded activity. If you can't do what you want (for example, change the activity type to something more granular/specific than Running, Cycling or Other; or automatically name the activity record not by location but by a user-preset text label, such as the name you've given a custom workout) with regular activity records produced and uploaded by your Garmin device, then you can't do it by performing a custom workout on it either.

That is a really bad suggestion and not how the rating system in the app store should be used.

Garmin will not use those ratings for the development. Instead use the suggested way to send your ideas. If enough people ask for it then it might come. https://www.garmin.com/forms/ideas/

Custom workouts is probably hard to add since so much relies in the system on the predefined exercises and how you build workouts. What probably would be easy is to add a few workouts named "Custom workout 1", "Custom workout 2" and so on that could be used. Even an alias system might be easy to add where all workouts could get an alias defined by the user. That would help a lot and then you could "rename" the already existing workouts and the custom workouts.