Thanks for the replies, all.
The videos were quite helpful. I know now that I didn't have my support-hand wrist straight enough. The main issue that I have remaining is that the right-hand side of thumb on my firing hand seems to be touching the slide release lever slightly. I don't know what that's going to do under recoil, but it may be a little while before I can get to the range. Is the position of that thumb a concern?
So, from what I am gathering, the thumbs-forward position seems to be the dominant position. However, it seems as though there are at least some authorities on the topic who are a little less rigid on that. Case in point:
https://youtu.be/XVvUHsFdHK4?t=360
At any rate, I'll continue working on the thumbs-forward grip and see how it feels after a few weeks.