Yeah currently it's a problem, but don't worry, with upcoming achievements update you'll earn quite a bit bathroom tiles, and I plan to add mission/objectives that provide practically unlimited bathroom tiles supply!
Viewing post in Bonus bathroom tiles is brutal
Can I suggest having this upgrade work retroactively -- as in, when you reset with one or more levels of Bonus Bathroom Tiles active, it adds enough tile reward to make your tile grand total match what you would have earned throughout the entire game if you had equipped the tile booster from the beginning? As OP notes, otherwise, you are permanently losing out on tiles if you focus on other parts of the upgrade tree rather than rushing this as fast as possible.
That's a very good suggestion. However as you noted having one shot rewards combined with the fact that there's a dilemma on having regular upgrades or tiles multiplier first creates a big unbalance. So fixing missing refund tiles only fixes half of the problem. What do you think, should I remove tiles multiplier and increase one-shot rewards progressively? (P.S Upcoming achievements update will bring extra tiles income)