While I live on the actual route (near Melrose) I’ve never ridden it beyond Yetholm, but the rest I’ve ridden in every season & condition.
The route from Yetholm to Melrose is all rideable, I did it a couple of weeks ago, but, it’s not a good time of year to do it as it gets so overgrown (field edges & nettle alleys). Best time is midwinter with a frost.
Bike-wise, an MTB is fine with an HT ideal, a gravel is doable (but I’d skirt the climbs near Morebattle & the Eildens).
There are a couple of stile/kissing gates that’ll need a lift at, but as it’s Scotland, all legal for a bike.
Shop/Cafe in Yetholm and Morebattle, Cafe at Woodside (near Harestanes) and Shop/Cafe in St Boswells.
Doing a loop I’d be tempted to park in Morebattle then ride St C until you meet Dere Street this side of the river Teviot, then loop up Dere Street until you hit the Howham road, run towards Howhan turning left before Over Witton and cross country to Witton and back to Morebattle.
For the last bit parking at Harestanes running on the St C to Melrose and then there’s various ways back, PM me if you need any help.