I dislike that subclasses come so "late" for some classes as well, but It's never bothered me enough to try something like this. It'd probably be fine
shrug, especially if multiclassing variant option isn't allowed, or if allowed it was limited to "no more than one" or something so you limit the # of dips.
It seems a little odd that the backstories are they are brand new lvl 1 beginning their adventuring careers... and already have access to advanced techniques and knowledge.
I'm having this debate in another thread right now. They may be beginning their adventuring careers at level 1, but they could not adventure and live their lives as skilled professionals per the PHB/DMG/XGE rules for "Working".
That + Backgrounds tells me that Level 1 PC's in 5e aren't supposed to be fresh to the world/naive PC's a la the Hero's Journey.
They are fresh to the world of
Adventure, but not to their chosen profession (class) or how they grew up (background). In those things, they are already proficient enough to actually earn a living doing them.
EDIT: sure, you could play them as young fresh-faced people, but then I'd limit the background options in some way for skills/tools/etc. Dunno. There is a cognitive dissonance for me between the fact that level 1 PCs in 5e are able to make a living with their skills/abilities at a Comfortable lifestyle level and the classic "hero's journey" from fresh faced farm person (Star Wars, Wheel of Time, etc. etc, etc.), so I err on the rules side and assume older people as PC's.