the path from slig to glen brittle is rough but doable especially if it's been dry. i rode it ~20yrs ago. the further from slig you get, the better it gets, once you get Sgurr a Bhasteir / Bruach na Frìthe on your left hand side it gets better (probably the high point). it's a walked in path/no construction, far narrower and rougher than what you've seen on the slig loop.
once over in glenbrittle you a few choices, follow the road down to campsite, single track and quiet.
cross the road and up in the fireroads around Beinn Bhraghad, you'll get some elevation there and some fantastic views of the cullin ridge.
you could back in towards Sgurr Fheadain climbers path, reasonably smooth, taking a dive/plunge in the fairy pools, taking the path back directly under Bruach na Frìthe to rejoin your path in.
down the road to
1. one way climbers path across from the youth hostel, steep start but fine once you get going into coire a ghreadaidh
2. down to the camp site, path into sgurr alasdair, again steep start then fine, one way again, you'll get as far as coire lagan, a beautiful spot with big mountains all around you
3. down to the campsite and down a track to the headland on the left hand side of the beach
the road section on slig loop from slig to luib is the main road, not main road as we know it but still busy, get it out of the way first would be my recommendation, not for tiring legs.
the drive from glenbrittle to slig is ~20mins
lots of options.
glenb campsite in the evening with the soft light on those hills is very memorable