I wouldn't use the Regal line for trim. It's just not fun to apply, even for a pro. A better choice would be the Waterborne Satin Impervo or Aura. If you do choose the Aura, keep in mind that there is a small learning curve in that it is a very fast dryer, so working in sections you can handle is very important. Pro Classic from SW, Muralo Ultra and C2 are other good choices for trim paint. You can have those non BM paints matched in BM White Dove.

When sanding, the goal should be clean and dull. You are not looking to sand the previous finish off, only to dull it. You can clearly see when you run your sanding block over an area that the sheen goes away. That is what you want. I wouldn't say that you need to press hard, but apply some pressure for sure. I usually will do 2-3 passes with the sanding block over an area before moving on. For primers, you want to select a bonding acrylic primer (latex). Pro Block, Zinnser 123, C2 One are examples. Not Kilz2.

Yes, I did an entire door in the Regal the guy sold me, and I had to add Floetrol to even make the brush strokes look tolerable. I find Regal too thick. So SW Pro Classic and Pro Block or Zinnser 123 is a good bet? Do I need to sand after the first primer coat even with the original sanding? Also, do I need to get all the sheen out of every nook and cranny, like in corners and in between the existing brush strokes? What is the right kind of BM trim paint to use?

Do I need to sand after the first primer coat even with the original sanding?
No, unless the surface isn't smooth, then lightly sand with #220 grit.

Also, do I need to get all the sheen out of every nook and cranny, like in corners and in between the existing brush strokes?

No on the nooks and crannies. Use a sanding sponge edge to drag through groves, etc. The primer will adhere to that.

What is the right kind of BM trim paint to use?
If you're a BM customer, the Waterborne Satin Impervo is a good choice. It only comes in Satin sheen so if you prefer a semi-gloss, move up to the Aura or cross over to SW Pro Classic.

Michael

Paintguy, what's the deal on Kilz2?

Kilz2 is a stain killing primer that a lot of people use/buy thinking it is a general purpose primer or a bonding primer or a good primer for new drywall and it's not. I think because it has the name Kilz and Kilz is pretty much the undisputed king of stain killing oil based primers, people have some trust in the latex version that is unwarranted. Plus, I guess it is easy to get too, but there aren't really any painting projects where Kilz2 would be considered a good primer for.