I’ve found Keith who owns and runs Mugdock cycles to be super helpful and friendly. I’ve had him replace my chain for price of parts plus a coffee and a chat. He also stripped the seatpost on on my then new road bike (not bought from him) and carbon pasted the bizarre infinite angle adjust mechanism so it wouldn’t drift every time I hit a bump, at no charge. Never had issues with them, and they always have a massive queue of bikes waiting for a service, so I can’t see them being too in-competitive. When I frayed the gear cable on my roadbike a few months back, he gave me a couple of new cables for free.
As to the trails, I love Mugdock. It’s my regular haunt, granted I’m no jump/drop hunter. The best trail, as far as I’m concerned, starts just outside Mugdock in Milngavie, runs past the Kyber Pass and along the West Highland Way out to Dumgoyne and the distillery, along some lovely single track, then up behind the distillery, through the forrest to Strathblane, and then a short road trip into Blainfield, from there up into Mugdock via the stables, then to the visitor centre for coffee before heading down Rosie’s Road via Mugdock and back along the WHW to Milngavie. It’s a great loop – known as the Dumgoyne Loop. Worth a look if you ever visit Mugdock for a cycle.