Hiya,
I'm glad you were able to make the device grant work.
I looked in the issues list and found this one: https://github.com/FusionAuth/fusionauth-issues/issues/248 which seems similar but maybe not exactly what you are looking for. If it is, please vote for it.
If it isn't, I'd encourage you to file an issue explaining exactly what you'd like, so that others can give feedback. We can't commit to timelines or implementations (unless you engage us for support or professional services) but definitely look at the number of votes for features when planning our roadmap.
At this time I'm not aware of any plans to store/display how many devices are linked to a user, which means that building it out in your codebase is probably the best path forward.
You could possibly leverage the user.data field and update it right after the grant is complete. This would give you the ability to search that in Elasticsearch.
Hope this helps.