A lot depends on your appetite for risk, plus your appetite for water and consumables.
I’d be a lot more likely to head out with a multi tool, tube and pump if I was doing something fairly public like one of the major bridleway routes (TPT etc) or towpath type bimbling. I guess I’d include trail centres in that.
If I’m going somewhere I expect to hardly /possibly not see anyone then I want to be able to be s bit more self sufficient just in case it all goes wrong. That means first aid kit, silver blanket, more water than I expect to need, a jacket and hat if I’m not already wearing a jacket, and possibly a bit more tool coverage- certainly spare brake pads, spare gear inner, spare hanger, OS map covering area and a compass. Relying 100% on a phone or Garmin is a bad plan. If I was going far enough out I’d probably start including stuff like a bivvy bag, some extra food and means of starting a fire / small Brukit type stove. This will all fit into an average size Camelbak. (The stove is starting to push it and possibly wouldn’t) Nobody wants to need rescuing, but it’s only a big crash or mechanical away, especially if riding solo in remote places. Phones are great, but you can’t always guarantee coverage so again if I’m going to be solo anywhere remote I’ll let someone know where I was going and when they can expect to hear from me again.
I sound like Mr Paranoid! 🙂