I did the following itinerary over two days last week, and these are my observations:
Day 1
06:22 Newcastle - Edinburgh. 5 car 801, fewer than 20 passengers on the entire train, all of whom apart from myself were in standard class. Pre pandemic this service carried enough passengers from Dunbar to fill standard class on a 9 car HST.
09:28 Edinburgh - Montrose. 4 car HST, roughly 75% of seats in standard class taken, didn't see first class.
12:21 Montrose - Dundee. 3 car 170, roughly 75% of seats in standard class taken, didn't see first class.
13:34 Dundee - Leuchars. 3 car 170, roughly 50% of seats in standard class taken, didn't see first class.
(slightly off topic, I did a bus return from Leuchars to St Andrews during this gap, and had both buses to myself)
15:20 Leuchars - Edinburgh. 4 car HST, roughly 90% of seats in standard class taken, didn't see first class.