Honestly, I think an Apple Watch (maybe the very affordable Series 3?) would be your best and most comfortable option. You can have your activites tracked by Apple's own first-party Activity app, which is likely the most privacy-conscious option.
I use an Apple Watch for cycling and several other activites. It isn't a necessity at all and one could debate the purchase for ages, but once you start using it you probably wouldn't want to miss all those small and frequent conveniences it provides in addition to the rock solid activity tracking.
I use an Apple Watch for cycling and several other activites. It isn't a necessity at all and one could debate the purchase for ages, but once you start using it you probably wouldn't want to miss all those small and frequent conveniences it provides in addition to the rock solid activity tracking.