I’m fairly new to Exmoor riding despite living in the same trail-blessed county. Honestly, the climbing is quite taxing. Very easy to get over ambitions with mileage. Plan a couple of escape loops.
There are many exciting footpath descents from Selworthy Beacon; if only we could legally ride them! I say study the OS map so you can see exactly where they go, that way you can avoid accidentally including them in your route. I thoroughly recommend the Periwinckle Tea Room at Selworthy for coffee and cakes.
Also, trails around Horner are very interesting too. Snaps tipped me off about the descent from Webbers Post to Horner.