My off-road commute is muddy and we have no showers at work.

Can I buy leggings or over trousers that will not drench my legs / under-leggings with sweat?

I have some Scott winter tights, that have soft shell shin and bum panels that are more than splash proof, with usual fluffy lycra elsewhere.
Seems a good comprimise of weather resistance vs sweaty.

I also just wear shorts as much as I can.

Stolen Goat orkaan (sp) tights if my top is anything to go by.

Enduro singletrack trousers very good

I’m in a similar position and it was fixed by making my bike as ugly as possible with full mudguards.
They work.
Plus they instantly make your bike less nickable.

This is as dirty as I got last year.

My understanding of your goal is not to sweat/overheat.
Not have dirty legs.

What about wet but clean legs? Singletrack trousers
Dry and warm – Costelli Nanoflex?

Full mudguards stop the spray from below. Rainlegs deal with the worst of the rain from above. Work OK. What they lose from not being full cover they gsin by being ventilated enough you don’t get wet from sweat.

irc’s recommendation is borderline cowboy chaps 😉

Just wear thick tights. You’ll be fine. No need for waterproofness.

Endura Stealth nothing at all compares in the waterproof and breathable stakes IME

Dont get the fleece lined ones as they are too warm.
Mudguards and MTFU is way cheaper

For my off road commute I use bib tights but with knee length SealSkinzs and Altura semi dry 3/4’s. It’s not often it actually rains on the commute and they deal with the spray well. I use a rear mudguard and a crud catcher and an old inner tube wrapped around the top of the rigid forks to work like a neoguard.

Even for trail riding in the wet, I use Berghause Paclite xtreme over trousers, with a mbuk freebie buff / tube thing as a ‘ gaiter’ on my right ankle to stop an expensive over trouser / drivetrain interface. Even on long rides, no leg sweatiness.
For what it’s worth I’m usually the bloke wringing sweat from the pads on my helmet, and wish that hydration pack venting was better,too.