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.
Day 2
09:44 Haymarket - Bathgate. 6 car 334, roughly 40% of seats taken.
11:41 Bathgate - Glasgow Queen Street. 6 car 334, roughly 40% of seats taken to start with, increasing to near 90% by the time Queen Street was reached.
13:04 Glasgow Central - Ayr. 3 car 380, standing room only between Paisley and Troon, roughly 90% of seats taken for the rest of the journey.
15:25 Ayr - Glasgow Central. 3 car 380, roughly 90% of seats taken
16:40 Glasgow Central - Carlisle. 11 car 390, roughly 70% of seats in standard class taken, didn't see first class.
18:39 Carlisle - Newcastle. 2 car 156, roughly 75% of seats taken, same as was common for this service pre pandemic.
Most of these services, to me at least, seemed to be near pre pandemic levels. The very notable exception to this is the LNER service. I also saw another LNER service during the course of the day at Montrose, where I counted 10 passengers on the entire train, on another service that pre pandemic was normally well loaded. Looks like the general public have started ditching LNER en-masse.