I really feel for you, it’s such a difficult process to navigate. I think it’s also hard to offer any guidance as everything is so different for everyone.
All I can offer, looking back, is to try to break things down. Maybe move everything from one room so that you have a totally clear space - I did this with mum’s things.
With that empty room, I then stuck bits of paper on the walls to indicate donate, skip, keep (me), keep (sibling), unsure,etc.
Even the pieces I decided to keep for me, it’s taken well over a year for me to integrate them into my space - I wasn’t ready until very recently.
I don’t think I’m explaining it very well, but am trying to say - don’t rush things, but don’t hold on to random stuff. The items you do decide to keep, don’t feel you have to find a p,ace for them immediately. And, those items you choose to donate, take comfort from the fact that your dad is helping others. Because I do gift aid, the charity I donate to send me an email every tax year to advise how much my donated items have raised and how much extra is from gift aid.
Aside from the above waffle, I really want to acknowledge your last sentence. That feeling is so intense and completely understandable. Just know that this board is here for you.