I have HD and a life time struggle against it. There are various sub types. I'm 2nd generation with a parent who was a squalor hoarder. It's a MH disorder that no one chooses to have, and is very hard to overcome even with good levels of self awareness. It generally worsens with age and other circumstances when there is little to look forward to. Attempting to start tackling it very late on, can be a death knell.
My sub type is an organized hoarder, possibly because of my childhood. Like with like, tidy, everything categorized and kept in a way it's easy to move and rotate to clean. (organized hoarders tend to need clean as well as tidy)
In case this is useful, I've successfully worked with other types of hoarders and go in with an 'I'm not here to get rid of stuff.' The aim is to get them back a level of control, order and cleanliness. In the process I know if it's is done well it will also result in them deciding to shift out some stuff, but that's not on the table until they put it there, then I facilitate it and ensure it works well for them.
'Organized hoarding' automatically helps overcome some of the more serious issues around general hoarding, and can be borrowed from by all types. (doesn't cure HD, but creates better environments)
So I'd advise you to head there in encouraging your dad to create a safer cleaner environment for himself, without hustling to get rid of things. You may be surprised how getting people organized without pressure to shed stuff, actually can cause them to decide to let go of some of it, especially when it's grouped together. They can see they're one person with a max of five visitors, who owns cutlery to feed 100 and they are likely to have only a few years at most left to entertain again. Being positive and keeping things light and getting to a point where they can laugh at things, can go a long way towards internal change.
I'm quietly glad to hear you're going to put the relationship first. Getting it cleared professionally when he's gone, can actually be done very easily and quickly and without stress. Fees for it should come out of the estate.
I can also tell you from the other side how heart breaking it is listening to older people with HD who have realized their children want them to loose the last few good years they have, going through misery throwing away what they want to keep, so their children can have an easier time getting their hands on the inheritance they feel entitled to, at the expense of the bequether's happiness.
It can also backfire.
One elderly lady become absolutely desolate as her family cheered on her spending her last year disposing of all that mattered to her. They consoled her, and told her how good she was being, and how relieved they all were for her.
She did it and with help, got to a point where it was a pretty normal home, but was so deeply saddened on may levels. She died as we got to the last leg.
The family were pleased so much had been achieved. But then furious to discover she'd been so hurt by their charade she'd changed her will and left the family home to a charity. They spent way more on legal appeals than it would have cost for clearance once she was dead, and lost. Her will was upheld.
It's very late on for him to try to elicit change without causing misery, so I'd say hold onto the good relationship you both have, while encouraging safety and a reasonable level of cleanliness and organization that is genuinely about him while he lives, and his quality of life, and while it's all a pain in the butt, you will come away happier in the long term.