You can skip the HR monitor. If you needed it you can add it later.
The advantage of the mapping functions is that it allows you to follow or create routes. For me it meant I got to a lot of places by the better routes. Note you will need a SD card with map on it, I’d recommend the OS one. Route mapping software also uses your SD card map. The basic map only comes with major roads.
Strava and phone is accurate enough. Why spend if you don’t need to?
I’ve had 800 for some three years and loved it. I’ve just bought a 1000 as I want the ability to upload my data to iPhone whilst I’m on holiday. I also use the mapping functions on off roads.
Should you buy? Depends on what you want to do. Plot and follow new routes to explore on off road, train, follow others routes etc from Internet then yes. If it’s to be just to upload to strava for the odd ride, stick with your phone.