Another approach is to adjust the "introduction" to the battle. Don't just roll initiative and start swinging. Include the moment of sizing each other up. Give time for the threats, the bombastic introductions, and the "transformations" — igniting the flaming sword, shapeshifting into a wolf, and chugging the potion of giant strength.
How things are set up in the game, you usually only have three options:
- Time to prep for the fight.
- Go into the fight swinging.
- Panic as you get ambushed.
An "introduction" phase gives you something between 1 and 2. You don't have all the time in the world to prep, but it gives you time to do something to be ready for the fight without making it seem like you're wasting your turn.
In this type of setup, I'd say you don't need to make all potions usable as a bonus action. You give them the opportunity to use the buff potions instead of letting them rot in someone's inventory for years, but without making it seem like you're cheapening their use.
For the mechanics of it, I'd say the Introduction phase lets you take one action to benefit yourself or your party, as well as accompanying dialog. (Same for the opposing side, of course.) Nothing directly offensive, but maybe allow for creating general area environmental hazards (bonfires, ice slicks, kicked over water barrel, etc) that have to be avoided, but might also be used during the battle.
At that point, I think the only question is on the handling of healing potions. I'm kind of on the fence about bonus action use of them, but I'd most likely go with the idea mentioned earlier in the thread, of the potion giving max benefit if you use an action, but just a rolled benefit if you use a bonus action.
I tend to not find them very useful, though. With lots of magic or standard rest mechanics, they're kind of pointless. With less magic or longer rest mechanics, they're an interference. Honestly, I'd probably replace them with a grant of temp HP instead of actual healing, letting them act more as energy drinks than cures. That would make their place in society as a whole make more sense as well.