If we want this to be DoS 3, keep the system as it is.
If you want to make a Baldur's Gate game, well, you can literally just look at what they did and see how it compares, and then look to the rules of 5e and see if they have an answer.
So:
Step 1 - In BG 1 and BG 2, you couldn't swap out armor pieces (main armor, boots, helmet, etc) once combat started. You could sorta swap your weapons, but it did take in-game time. That would mean your character is locked with whatever non-weapon gear they had equipped at the start of combat.
Step 2 - In 5e, it takes an Action to 'don or doff' a shield, aka it takes an Action to put on *or* to take off a shield. That means swapping between a 2-handed weapon and a sword and shield would take an Action to do. Additionally, pieces of armor have various times to 'don or doff' (PHB pg 146). Light Armor 1 minute to put on/1 minute to take off. Medium Armor 5 minutes to put on/1 minute to take off. Heavy Armor 10 minutes to put on/5 minutes to take off. This makes swapping your actual armor nearly impossible in-combat....unless you have armor specifically designed to do so. The rules for 5e have special magical armors that are nearly instantaneous to put on or to take off. For example, Xanathar's Guide to Everything has a Common magical armor called "Cast Off" armor that just takes an Action to take off.
So, homage to the BG series says no swapping armor in combat. The rules of 5e basically say you can't, unless you have armor specifically made to do so (such as a suit of magical armor).
This is a great example of where Larian is going to have to make some real decisions on what kind of game they want to make. Sticking closely to the rules is the easiest (and in my opinion the best) route because once you do that, the game will suddenly start feeling a whole lot more like a D&D based cRPG, rather than just another RPG in the DoS engine.
