Start with all the stuff that is stored.
If you genuinely know it’s going to be difficult/take too long for you to sell it, then best thing to do is to donate it, it’s not being used and it could be given to someone who will get use and enjoyment from it.
The external storage, is it owned or rented? If rented, work out how much you’re paying weekly, monthly or yearly to literally fill it with stuff that you don’t have/don’t need day to day access to or a use for, and even if you aren’t renting it, you are still paying for it, leading on to next point.....
Every square inch space of your house costs you money, money to buy it, money to decorate it, money to maintain it and money to clean it, so make the things that use up your valuable square inch space “earn” their stay, if they don’t bring you a use (and as Kondo states, bring you joy) then it hasn’t earned its right to be there anymore.
I know it’s really hard to let go of good stuff and it can make you sick to think what you’ve spent on it, but it isn’t a waste, you got joy from when you bought it, joy out of it when you did use it, and now joy from it knowing you are helping someone else love and use the item and helping charity out.
If you are going to sell, then great, any money back is a bonus (just don’t spend it on more stuff 🤣) but it is a very long and laborious process to sell and will take consistent commitment until sold and something more costly than money - your time, something you will never get back.
Once everything you no longer need or use is gone, you are only left with what you do, so you can then organise this around how YOU/YOUR FAMILY live, you can read home ideas and organisation threads until the end of time, and whilst useful, they’re aren’t your home and if it doesn’t work for how you live your life then it will always be a problem. So organise your home in a way that suits you, your home, what you use and how you use.
Once you’ve cracked it, it really is easy to keep on top of and cleaning will just be day to day maintainence of your home system.
Don’t buy more storage for more stuff, again this is just more cost to have things not “earning” their keep.
Good luck