Is there an easy way to edit an existing Strava route?

Not in my experience. I've had to redraw a route before on Strava to do what you're trying to do. You could try exporting it to Garmin Connect and trying it there.

Edit:- this might be a workaround.

https://communityhub.strava.com/t5/strava-features-chat/edit-a-route-so-it-starts-and-finishes-in-a-new-location-along/m-p/16295#M2660

Another thing that annoys me about Strava is that the desktop version is almost like different entity, and doesn't seem to share much functionality, and looks quite different too.

Trouble is, I can’t figure out how to edit it. I am starting from an entirely different location but want to join the route I did half way through and follow it.

Surely this is easy?

Not really but what you can do is start from your new location, ride to where the route is (at whichever point you join it) and then fire up the old route on your head unit. It'll start asking if you want to be directed to the start, click NO and just start following the route. It will pick it up from the point you're on, even if it takes the unit a minute or so to work out what you're actually doing.

Another thing that annoys me about Strava is that the desktop version is almost like different entity, and doesn’t seem to share much functionality, and looks quite different too.

Yep, a baffling lack of continuity between them.

If you can export/import the file into Komoot you can change the start point quite easily.

I use Garmin Basecamp for this sort of thing.

Editing the start point is hard but editing the end point is easy. So, on desktop:

Reverse the route.
Edit/delete any way points from the end of the route to finish at your location.
Reverse the route again.

Download GPX. Edit in RideWithGPS. Upload back to GPS.

I've found before that my wahoo won't sync a new route when I'm mid ride, unless I end the current ride (and start a new one once I have the route). But if you've preloaded the route earlier you can just piggyback on to it.

One thing to look out for on reversing the route is that anywhere that someone has manually put in a section down a bridleway/path that Strava doesn't recognise as being official/usable is that when you hit reverse it will re-route around any of these sections so you can end up with a completely different piece of routing.

You can delete any waypoints including the start point, so just delete waypoints starting with the start point and then add points from the finish to follow the same route. Might be little time consuming if you have to use 'manual mode' but it's not too much trouble. Just make sure you are clicking new end points in, rather than dragging the end point, as this will create new waypoints which will keep you on the right route.

I've found it easiest in the past to just download the GPX off Strava, edit it in gpx.studio which has a bit of a learning curve but does let you just delete the front and back parts of a ride for example, then re-upload.

I can't see the problem.

This is on my phone from the website, not the app. Either edit that route or copy it first. Then zoom in to the start, click on the start, wait for the menu to appear and drag away from the finish. It's then easier to repeat zoomed out a bit.