OP - I see a lot of similarities in your life to mine. But with a couple of very important differences:
Mid-to late 40s, I'm the main breadwinner. 2 kids, oldest is 12.
If I am around, DH wouldn't feed a child, put a child to bed, think about bath time. If I wasn't around, he MIGHT do these things, but seldom well, and never on time. (this is in the past).
He can't (or struggles massively to) plan, organise or anticipate.
DS has recently being diagnosed with inattentive ADHD. We are 90% certain that DH has it too. We have discussed whether he should seek a formal diagnosis, and possibly, medication. We are not certain becuase... and this is where my life and yours diverge massively..... DH has worked extremely hard to overcome many of these challenges. He is not 100% there but the difference today vs years ago is notable. Some examples:
He worked in the arts originally. Stopped to get a better lifestyle. Tried various forms of more traditional office jobs which was a nightmare - was fired, made redundant, stressed etc. Became a SAHD for a few years and while we had some moments, worked hard to constantly improve in terms of anticipating DC needs, thinking ahead etc. For example, he was almost NEVER able to get himself and DS out the door - it just felt too overwhelming for him. Eventually he created a list of actions prior to departure and stuck to it religiously. Their world expanded accordingly. Booking/planning/researching activities felt very overwhelming so I'd start by doing some initial research, then encourage him to book it then over time, he'd get more comfortable and confident.
As the DC have got older, he has retrained in a one-to-one role, managed centrally and is excelling. Money isn't great, but he's still the primary carer, does all school runs etc. So it's fine for us.
Food - he's still not brilliant, but a) kids are old enough to remind him they need feeding and b) he has a small selection of options he is comfortable whipping out (some of which I've helped him to learn) and c) I try to accommodate this challenge for him by preparing/meal planning/shopping.
He has slowly taken on more actual workload at home AND more mental load. Some is the bigger, less often stuff like insurance, cars, dental etc, but some is more day-to-day focused like garden, bins, general tidying, bedding, laundry, home IT support
etc, Each one has taken effort - often initiated by me then continued by him. He now regularly does the shopping (I do all meal planning and shopping lists though) something he found completely overwhelming and would take him 3 hours previously, and is responsible for most of the laundry-related tasks (I'm still more likely to realise washing needs to be put on, and as I WFH, it's easy to do. He however will absolutely hang it up, does all the folding/sorting/putting away etc). He has taught himself various DIY skills so he can do all that.
It hasn't been all smooth sailing to get here, but I can honestly, hand on heart, say that today my resentment levels are almost non-existent. There are the usual small niggles (his inability to put things in the dishwasher! :) ) but between the number of hours he's now working, the childcare he does, the volume of chores I do NOT have to do... I feel pretty good.
The difference is that in the almost 20 odd years we've been together, DH has consistently and regularly acknowledged that he is poor at certain things and then done his best to adapt and improve.
This is all very long-winded, I apologise, but I think the key issue is not whether or not your DH has ADHD. The issue is whether he acknowledges that he is shit and that he should put the work in to fix things.