You've already outlined some good alternatives, mainly pre-exhausting the pecs before getting to the lighter dumbbells. Don't bother further with any injury-causing movements as the problem is likely to continue to recur. FYI as you probably already know, anyone with shoulder probs shouldn't be bothering with free weight BBs at all in lieu of other alternatives like good machines, DBs or cables.
If you avoid pre-exhaust and go straight to the lighter dumbbells, you can increase the intensity using the same weight by supersetting them with another chest exercise, by adding partials or rest-pause reps after reaching failure, including negatives or by reducing the rest times between sets, to name a few. Super-slow may appeal to some but i've never found them effective or attractive; mainly they're gruelling.
Another possibility would be to do incline flys, DB or cable, instead of DB presses. A very good alternative to DB presses. Or use these as one of the superset combos with the existing presses.
All of the variations mentioned would make sense-try each for a while, keep those that are effective for hitting the desired areas, cycle each in and out over various workouts along with any other variations you find.