Not my area of expertise but density is the wrong concept to be looking into remaining equal, its pressure. The amount of helium gas you pump in to the balloon creates a certain outward pressure that’s at equilibrium with outside air pressure, that’s why it doesn’t collapse (if you were to place the balloon in a vacuum then it would expand and probably burst, that’s usually what happens when a child loses one and it escapes higher up in the atmosphere to lower pressures)
The atmospheric pressure simply isn’t strong enough to overcome the outward pressure.
Bonus point: density is actually the reason why a helium balloon floats, its less dense than air so it wants to float up. Much like a rubber duck or ice cube in water.
I’ve found myself in similar situations. However I haven’t taken full three weeks off; I think my longest has been about 10 days.
I’ve always relished getting back to it with a clear mind and often a well thought out plan and it feels like the features flow faster from the fingertips.
Never fight taking a bit of time off, it grants perspective and lets you decompress, don’t think that can ever be a bad thing (given there’s nothing you really need to be doing in that time to make sure you’ve got a steady income)