In reply to Hannah Mitchell:
I have Garmin 310xt, 910xt, 920xt and Fenix 6 - the 3 earlier ones are Triathlon/multisport style watches, the Fenix 6 is a 'smartwatch' with all the features to compete with Apple and Suunto etc. Pretty expensive but bought for maps and 35hr battery life - the previous models could not manage a Bob Graham or similar at my pace... The Fenix has optical HR but it is useless when running at pace, usually over-estimating by getting confused with cadence. This screws up the training advice that the software offers and shows a gentle jog as a maximal effort sometimes. I use the Garmin HRM-Run strap which also offers running dynamics - cadence, L/R balance, 'bounciness' etc - for all proper running. Strava will link to Garmin Connect, I don't use it though (it's for the young folk on social media I believe...).
All the smartwatch options are there, most of mine are turned off (floors climbed, calorie intake, move now etc) but I do look at sleep quality, resting HR, training status and predicted times for standard race distances - all close to what I could achieve ten years ago!
Fenix 5 and earlier models can do a lot of the same for much less money but without the long battery life, might be worth seeking out one of these but you will have to budget for a proper HR strap at £60 or so.