Have been using latex tubes for 3 years now and have never had a problem with tears at the stem base or problems inflating them.

I don't do any tricks, just use some 'Fourier' branded extenders that aren't expensive from Amazon, and they've never leaked air from the junction, ever. Even when I've flatted on the road and swapped the extender over on the road, no problems with leaks,ever.

I don't even use that screw-on circle (retaining nut?) that holds the valve in place when you inflate. Sure, the valve will depress a little if the tube is totally flat, but you can just put a finger on the opposite side of the valve against the tire surface to give it a surface until it inflates enough to hold on its own.

My Fourier valve extenders are also not perfectly lengthed. I just bought the really long ones that reach for my 88 deep wheel, but I use them also on my 50s where they stick out really far and def aren't aerodynamic, but still work fine.

I've been using Silca latex tubes. Multiple tires, GP5000, Gatorskins, etc. No problems with any of them, even when flatting on the road.

Would something like the Silca Speed Shield do the trick?

This. It's the same as every over tube I've ever inflated. Latex tubes aren't as fragile as some people make them out to be.

Press against the tire where the valve is, problem solved.

The simplest solution is always the best. As incredible as it may sound, I never considered simply depressing the tire with my finger to limit the valve movement.
Thanks Gang!

Even before doing that, it's really natural to just let the ground serve as the surface to press on the other side of the tube. I mainly use my finger when I'm on the road and not using a floor pump, but a minipump so it's hard to get that thing started in certain positions.

Silca valve extenders are externally threaded so you can use the nut