I’d label each task, figure out the cost of each, the timeline of each, and the impact to boarders during the reno/upgrade for each.
Is the reno/upgrade cosmetic (ie. tack room)? Is it going to create a healthier environment for the horses (ie. covering/replacing stone dust aisle)? Is it going to be nice to have on the rare occasions it’s needed (ie. wash stall)? Is it going to be a draw for new clients (ie. arena footing and cover)?
For me personally I want horse health first - so that stone dust aisle is probably my #1 because of the dust, poor ability to be kept clean which means mold growing. I have a horse with heaves so that is colouring my answer.
I want to ride so the arena footing and cover might be my #2 depending on what it’s like now, and how long it’s going to be unusable during construction.
The wash stall might be #2 as treating injuries, cleaning up and similar uses of an outdoor wash stall will be much more pleasant indoors during the cold and wet season. Rarer usage to be sure, but beats a prettified tack room (see next point for caveat).
If my tack/gear is safe and secure, and there are things that could be done to improve the environmental health for my horses, I would be unhappy/displeased/annoyed/irritated/frustrated/etc by the “waste” (this is personal perception!!!) of resources spent on cosmetic upgrades to the tack room.
Timeline is definitely a factor to consider. For example the arena upgrades might drop lower in my priority list if it’s going to impact my riding for weeks.
That said, I really think you should be listening to your boarders. You could ask formally or in a casual “I was thinking of finishing that indoor wash stall or maybe redecorating the tack room - what do you think?” sort of conversation.