...he would be totally focused on learning the LT position by playing it in every game. And more and more as time goes on..., but yes..., he would be learning from the bench too (from the best). And though Whit is playing at a pro bowl level now... I'm 'assuming' he will not be by the final 1/3 of the season. And yes, you did note that switching players in and out is not ideal. But I think sometimes you make exceptions... nothing carved in stone these days.
I'm also not just looking at this season. If Boom doesn't get experience at his expected position next year... I think the Rams could be in a real fix. We don't have a #1 pick in the 2021 draft..., and we cannot afford to pay a top free agent LT next season.
I think my plan (others too) could work.