Another litterbox problem

We have a busy cat household of four cats.
Our eldest is 11 years old and very territorial (owning a dozen or so gardens) though I am beginning to think he may have peaked in his power.
We have a female who is 4 and two males who are just over a year old.
All cats have been neutered.
After a few scuffles the eldest cat remains king of the house and tolerates the two youngsters and our female, mainly due to being a very outdoor cat who uses the house like a hotel. In the last few months he has started to have a few urination issues, despite us still using Feliway in the house.
Though neutered he seems to want to spray/urinate up against walls or on things we leave near the door (bags, boxes, etc).

The other day we caught him standing in front of the litterbox and spraying urine over it to the wall behind.

The vet has said that cats often urinate on floors to show you they have a problem, but walls are generally territorial. He said that our cat doesnâ€[emoji]8482[/emoji]t seem to have a problem urinating but has given us tablets for cystitis, just in case it is that.

We have separate litterboxes for the cats, but weâ€[emoji]8482[/emoji]re not 100% sure that one of the cheekier young males hasnâ€[emoji]8482[/emoji]t use the older catâ€[emoji]8482[/emoji]s litterbox.

Does anyone have any clues as to causes or prevention? Has anyone encountered this problem before?

IMO I would not give meds for UTI/crystals unless the cat was checked out. Am surprised your vet did this without checking first.

Anyway, have you tried the covered litter pans instead of open ones? Some cats would rather stand up more when peeing and would then hit the walls, outside the pan.

How many pans do you have? He sounds like a macho male and even if its more pans, he still may mark territory.

We curently have 4 litter trays but only two are in use at any one time. The young males share theirs and the oldest male has his own.
We've just purchased a covered tray but as it looks a lot like a carrier he won't go near it (it reminds him of vet visits).
The female tends to go outside (mainly in neighbours gardens!!!!).