Sanctuary is a life saver for that encounter btw. Prevents any direct attacks on someone.
Everyone has done a good job covering the pros/cons of what you sacrifice at each level. I'd just like to mention that the specific classes can matter too. For example, I like multiclassing monk on everything because punching is fun. In a situation like that, getting tavern brawler at level 4 is my power spike, so I stick with the main class until 4 then throw some levels in monk. It's possibly to find pretty goofy combinations between classes and only 100 gold to respec so there really isn't harm in messing around and testing builds out.
Realized that after reading it again oops. Removed my first sentence in that case.
There's some additional late game companions in act 2 and 3 but not too much. A lot of casters (including druids) will also gain elemental summons as spells in later levels. The summon itself can be a tank for you. You can also respec characters at Withers in your camp. You want to be careful changing origin characters classes, but subclasses are fine. Clerics can be great tanks, especially in the life domain. For example my current character is a cleric/monk with an armor class of 24, almost ever attack misses them.
EDIT: removed my first sentence, it mentioned the class of a later game companion which may be considered a spoiler to some.