If you are comfortable with a pack, I would go that route.
I personally do not like wearing packs. I have done 12+ hour days in the desert with just bottles (70 miles and 12000’ climbing) on my mountain bike, using pre planned areas to refill my bottles (my car and restaurants). I have a very small pack I use for running, but even then I only use it when necessary.
Nothing wrong with putting mix in your bladders, just rinse it out as soon as reasonably possible after you are done.
On my big bike (my preferred bike), I can add a couple extra bottles to the SWAT bibs for more water. I will do the same with a jersey pocket on my XC bike (which has two cages). Pretty rare I am on a ride where I can’t plan a stop to refill my three bottles, even if it is a stream or snow or something. I set a FKT on a local MTB course (50 miles, 10k’) a couple years ago pre staging bottles on the course to swap out the morning of, picked them back up after I was done.