Oofta
Legend
The infiltration of the keeps of the Storm Giants does not require everyone to be the same level, which seems to be your big sticking point. Maybe the first level character that is in tow with the mostly 6th level party is the key to overcoming some arcane locked door - only a fresh drop of their blood will open the door that gets the party to the McGuffin which is the goal of the infiltration. See what I did there? I came up with a fun way to include that player. The other PCs better do their best to protect their first level companion, or the quest will fail. Maybe that’s not your style of game, but it doesn’t mean it can’t be pulled off at most tables. The campaign can be long enough to give that PC a chance to shine in combat against tougher foes... if they survive... just like any other PC.
The storm giant's keep is one simple example. In others I've had PCs going into areas where they were expected to take on the leadership of the enemy army, or infiltrate the cult that had the power to transform into demons, etc. I gave a sample encounter for level 10 party a while back and explained why I didn't think a low level PC would survive. I can go back to my notes for the level 18 party that had to replace a PC after a heroic sacrifice if you want.
So if they just need that fresh drop of blood, great. But what happens when the flying nalfashne that likes to ignore the front line and go for the guy in the back (stated tactics in the MM) pops in? What happens when the invisible wizard casts chain lightning? There's little or nothing the group can do. In a post by he-who-does-not-want-to-acknowledge-my-existence-any-more, he talked about a low-level fighter in the back who eventually ran up to help. That's great. But why was that low level fighter never targeted with an attack? When I asked I got something about two PCs going full defensive (unless I missed a post but he refused to clarify). So? Two tanks are going full defensive. At best the low level fighter has a +2 to AC. If I were a PC in a game, I'd ignore the tanks and target the enemy I could hit. I may even provoke an opp attack to do it. Why would the enemy do any different?
Simply put, there's not much you can do to protect a low level PC short of having parallel encounters and plot armor especially once you throw in AOEs. If they're having parallel encounters and only joining up for social and exploration time, it feels like separate campaigns where I'm super-leveling the low level guy artificially just to get them to a point where they won't die. If I'm doing that, why bother?
I can re-post my sample encounters from a random level 10 game if you want. Tell me what the party would do to protect someone with a dozen hit points or less other than leave them home at the tavern. I would like to know because I keep hearing a lot of claims. Maybe there's just something I'm missing but 5E doesn't have a whole lot of ways to defend others from attacks other than plot armor.