Yes, it’s usually better to do services, like central heating/water and rewiring, before decorating a flat - if it makes financial sense to do the works in the first place.
As I say, you’ll have to weigh up the numbers, to look at payback periods, potential property value uplift and how long you expect to be in there.
If it’s less than five years, I’d probably look at keeping things as they are, unless the tank is severely restricting your options in your bathroom.