Ultracycling

Fuck tubeless
Fuck deep section rims
Fuck low spoke counts
Fuck any weird spokes that most bike shops will look at and say "what's that?"

Hope 20five pro 4. Faultless.

Is there a wheelset or rim that is a clear go to for mixed terrain long distance riding?

Run tubless.
Get something hand built
25mm id
Not deep
Carbon rims if you can afford otherwise alu abd maybe dt xc rims
Dt hope i9 ck hubs as budget
Dt or Spaim spokes

Loads of people use tubeless for TCR, maybe it's 50:50. Sometimes it goes wrong, most often it doesn't. If it does it's a pain but not race-ending. If it doesn't it saves you time. Basically it's not a major decision to agonise over.

Re rims, Kinlin XR-31s are pretty good. I've bounced my set out of loads of potholes with no issues. But really, most modern, deep-ish section rims are strong enough. I can't recall hearing anyone on TCR have a bust rim, apart from Ultan Coyle who got hit by a car, but that hardly counts. If it did happen would most likely be box section like an Open Pro as they are not as strong as modern wheels.

If I was getting another Alu set built I would go for Aforce AL33 as they seem to be the fastest and strong too.

If I had disc brakes I would have carbon. In fact that is the reason I would switch to disc brakes, so I could have carbon rims for something like TCR without worrying.

How common is it for people to be running tubeless on the start line of say TCR?

Sure - I was answering this question, then I rolled on to rims without resetting. For TCR I wouldn't optimise for gravel, I'd optimise for road that could survive gravel - so a differnt question.

@skinny has 1000x more experience of rims for gravel so listen to what he says!

I have Halo Vapour GXC in both 700c & 650b (tour version with 36spokes) and so far they've been faultless. 3 or 4 different tyres have all setup tubeless with ease and hold air well when not in use for weeks/months at a time.

When the 650b are on it gets used like a mtb and they're still solid and true. It was between these and some Hope XC/Gravel wheels and the Halos edged it as I prefered the finish and rim profile, no regrets so far.

If you like a quiet freehub these wont' be for you though!