I am .. in much the same position.
Not looking forward to when Mum's partner passes away and I have to decide what to do with all her stuff which is still in the house he lives in right now!
But. Just be clear with yourself. How much space do you have? How much stuff? How much definitely has to go somewhere else? Do you have the money to pay for storage? Being honest with yourself, would you just be throwing money away on storage fees for so many decades until you die and the kids throw it all out, because they don't have time to go through it?
I think the key thing is, START.
What is in this box? Why should I keep it, or, not? Should I keep 1 and take photos of the rest?
What you keep, ORGANIZE AND LABEL.
Even if you go through a box and keep half, now you have more space, and what you kept, you now can put your hands on when you want, and your kids will know what it is if you pass away.
You can probably make more space by boarding the loft or whatever, but again, think - do I want to pay all that money? Or will I feel better if I donate and sort all of this stuff?
Some people will have spare cash but no time and loads of stress and, for them, paying for the boarding and sturdy lidded transparent boxes will be the least worst thing right now. There's no right answer.