I still have nightmares about clearing out my childhood home after my father's death and it was many, many decades ago now! My dad had hoarding tendencies, my mum had been in bad health for a good few years before he died and so hadn't been able to really keep on top of much of it and it was a large house with many, many hidden corners and cupboards so it was a big, big job! But, to be honest, although it was hard physical work at the time, also emotionally difficult, I did also find the process of readying the house to be passed on to another family, hopefully to love it and bring up their own children there was quite cathartic, although letting it go with all the memories etc was really sad, I found the state it had been allowed to slip into sadder, plus I actually found getting stuck into a solid, practical task quite helpful with the grief.
Things that help that you could think about doing now, I don't think it's reasonable to expect your mum to actually sort it all out, it's probably overwhelming for her esp if she's a bit frail. But what she could do maybe is identify the most precious items, make a list of what they are and where/which room they are in in the house and (if this isn't too morbid) note any particular requests about who should have them after she dies. This latter point would have helped me enormously because a bit like you, our house had become the repository for a lot of family 'heirlooms' (of dubious value and quality mostly!) and the absolute worst part for me was the family arguments and stress caused by my relations saying things like 'oh your Mum always said I could have Aunty Ethel's jewellery box', 'I'm really attached to Grandad's books, don't throw them away', when I didn't know where in our huge, disorganised pit of a house these things were and whether they'd been promised to someone else or already given away or what. In some cases the recipient was not even prepared to come and help find their cherished items themselves and take them away, they wanted me to search, find and package their items and somehow transport them to them safely, all paid from my own pocket of course! One case my uncles actually said to me that I absolutely couldn't get rid of a huge ugly mahogany bookcase because it was valuable and an heirloom but neither he nor anyone else in the family wanted it in their house, what I was I meant to do, just take it to my house and keep it on his say so?! It turned out to be worth about £50 btw!
So yes, I think the best tactic is to focus on preserving anything precious or sentimental you really want kept, and then the best thing TBH is to either turn the house over to professional clearers or turn totally dispassionate yourself and hire a skip or van for tip and charity shop runs and empty it quickly without doing too much picking over of individual items and sorting. I know this is probably wasteful but is the best way to get the job done without it taking a zillion years and absorbing your whole life and energy into it.
Would your mum ever consider moving out - your childhood home may become impractical for her to manage in her later years - my mum moved out and sold ours after my father's death and it was 100% the right choice for her. Personally I am a huge fan of sheltered/retirement flat complexes because they are so much easier for an elderly person to manage (bills etc are covered in your service charge or rent), extra help is usually very easy to arrange if needed, the manager of the site can usually advise and they may have on-site carers, cleaners etc already and a canteen or cafe for meals, plus there's a ready made community of similar aged people which can do wonders to combat loneliness. I know that's not the question you asked but I think it would be loads easier to sort and clear the house while your Mum is still with us so something to consider??