Yeah, both seem to have their ups and downs
more water volume = more dilution of pollutants = less dire need of water changes
set up now, chance things could go wrong while you are gone and lose everything/a lot.
less water volume = higher chance of it needing a water change (possibly before you return, but you mentioned you have let them go for longer without)
leave as is, but less water volume = less dilution so higher chance of polluting the tanks killing off fish if the tank sitter overfeeds
Take your pick.
With 2 weeks before you have to leave, I would think (If done right! kind of hard to mess things up, but that is possible) that is enough time to figure if the 100 Gallon route was set up and stable enough to trust leaving.
I've done similar with combining x5 20 gallon tanks into a 100 gallon all at once just fine and it was stable since day 1 of transfer, no mini-cycle or any spikes at all. Just transferred the fish (by no means a light bioload either) and only a handful of plants, brought over the filters and was smooth sailing.
Then again if you waited, you could do it when you get back (assuming everything survived during your vacation) and take your time doing so.
There are products that I have seen such as "ammonia pads" and zeolite chips that supposedly remove harmful traces of ammonia. I've never used them, but you might consider those as insurance haha.