help with transferring an unlocked Samsung S10e to AT&T

Once you switch the sim into the s10e the network with automatically register it as a new device. =]

   So to answer, thankfully with physical Sims it is as easy as turning both devices off before moving it over then rebooting the devices.

Your voicemail may reset so be sure to save any that are important.

a. physically transfer the nano-SIM from the old to the new,

You should switch your data first and configure your new phone the way you like it. Once everything is set up, and you have signed into all of your applications, verified all of your two-step authentications, through your old phone.

Then you can switch the SIM card which should be a nano SIM. If you're old device does not have a nano SIM you will have to have the SIM card replaced after you set up your phone at an AT&t store

b. move the micro-SD from the old to the new

Yes

c. using Samsung's "SmartSwitch" app, transfer all data form the old to the new

No, this is the first step ⤴️

d. contact AT&T and give them any info about the new phone.

This is unnecessary.  Your new phone will register on its own. 

And maybe there's more  need to do. So if you all have suggestions about where I might find help taking these - and any other necessary - steps, it's greatly appreciated. I've searched "transferring an unlocked S10e to AT&T", but no luck (so far)_

You are overthinking this.

thankss again! great help

just one more question - - before I pull the SIM from the old Galaxy Express 3, should I AT&T unlock it - in case I want to sell or trade it?

Yes.  It's always easier to unlock a phone as an existing customer