When there's no disruption, the Thameslink service is excellent. Unfortunately, when it goes wrong it goes very wrong and every service is affected, even those not going through the core.
I use the service almost every day, at all sorts of times.
Post-covid was the perfect time to introduce changes to the service which has had a fairly set pattern for many years (even GN pre-TL), but there are more trains now.
Excluding the Kings Lynn/Ely trains, there are 6tph over Welwyn viaduct. I'd split them 3tph to Peterborough, and 3tph to Cambridge, rather than the 2tph PBO and 4tph CBG that exist today.
I'd run the Peterborough services all stops to Stevenage, then they'd have different calling patterns - 1tph non-stop to Finsbury Park; 1tph Welwyn Garden City, Finsbury Park; and 1tph Knebworth, Welwyn North, Welwyn Garden City, Finsbury Park.
After Finsbury Park they'd all run through the core.
As for Cambridge, 1tph all stops Cambridge to Welwyn Garden City, Finsbury Park then core; 1tph Cambridge, Royston, Baldock, Letchworth, Hitchin, Stevenage, Welwyn Garden City, Finsbury Park then core; and 1tph Cambridge, Royston, Baldock, Letchworth, Hitchin, Stevenage, Finsbury Park then core.
As you can see, I'd remove Hatfield and Potters Bar stops, but replace them with 6tph (4tph Moorgate, 2tph core) Welwyn stoppers with much improved connections at WGC.