I'm not a BF counsellor and only 8 weeks into BFing my first baby but here's a bit about our experience in case it's helpful. Others (tiktok) who are real experts are worth waiting for on here to get their view.
My DD gets a bottle of expressed milk for her late (10.30) feed. I express this in the morning after her 7am and/or 10am feeds. However, I do always make sure to express at 9pm ish too, for several reasons:
1 - to stop my boobs becoming engorged and uncomfortable while waiting for the next feed
2 - to keep my supply up (so my body knows it still needs to produce the same amount of milk despite the 'missed' feed
3 - to build up a 'bank' of milk to freeze (although with thrush this isn't advised)
Expressing in the morning shouldn't lessen your supply for the day, if anything it should help stimulate your supply.
What was it that made you worry about your supply after morning expressing? It's natural for your boobs to feel less 'full' as the day goes on, but there should still be plenty of milk there! Was DS fussing at the breast at all? If not, I'd say your supply was probably fine.
If you would rather express at say 9pm you may find you get less, but a warm shower first and some breast compression during the expressing should mean you can pump enough for your DS to have as his late feed.
I have managed to pump pretty well at 9pm ish doing these things, although it has got easier now my milk supply is properly established (once we got past 6 weeks ish).
It'll get easier once you can store your milk for longer of course so there's no rush. Hope the thrush clears soon.