I'm sure I'm not the only person who tends to drift, or slightly shift weight to the trail side about mid-way up the backswing. I believe it's caused by my lead shoulder moving off the ball about an inch or so at the start of the backswing. There's a LOT of weight in the shoulders and head, thus I think any lateral upper body movement to the trail side automatically moves my weight distribution to 40/60 without even feeling it. Likely because my "brain" is used to that feel.
So "lead shoulder down" will help maintain a 60/40 distribution, but would a wider stance also help? Also, I saw a post about pre-straightening the trail leg at address (or nearly straight) to keep the weight distribution in check. This would also seem to coerce the trail foot to "bank" inwards as the club reaches the top of the backswing (which would keep weight to the left side), but could that lead to other problems, or is that a more "permanent" fix?
@GolfLivesMatter - There is some measure of tolerance for a brief transfer of pressure into the trail side if it's simply in reaction to the weight of the arms and the club moving back in the takeaway, but if it's an actual shift due to the head and upper torso moving back then the preference would be to not do that.
Presetting the hip turn and straightened trail leg at address - which I oftentimes refer to as the "Elkington Setup" since he deployed that very effectively in Tour events when he was transitioning to S&T - can be a drill for many but can also be a permanent install for those with limited mobility. I would consider you to be in the former category, since this would be something used to target and eliminate a chronic issue that you seem to be battling. However, if you find it produces consistently crispy ball striking and a desirable shot shape without sacrificing distance, then there's no downside to playing with it.
Here is a link to a lesson on "The Elk".
Happy New Year!!