Never too late. I think many parents hit these ruts I know I certainly have at times, especially when dd was going through her 6 year old fussy phase and I had a long commute and was KNACKERED!
My mum bought me a slow cooker! Bloody godsend!
"I do think they would eat toast for a year before they would give in" ohhhh I feel you! At one point if I'd let her (and it was sooooo tempting) dd would've lived on cheese and crackers! She took a notion for a particular kind of cracker and she's always loved cheese (as do I but you can't live on it!)
I too was raised in a "there's 2 choices in this house at mealtimes - eat or starve" house and quite honestly a lot of the time I starved! I never liked meat and turned veggie at 16, a combination of eating food I liked and also stomach issues I'd had all my life all but disappearing (I genuinely believe I cannot digest red meat which we were given a LOT at home) meant I started gaining weight which my mum was really surprised by. She is now in her 70's and when discussing she says now she wishes she's let me go veggie earlier but she was also a working mum and had a lot on so I understand why she didn't want to either faff making 2 meals (much harder in the 80's when there was precious little quick, easy, veggie stuff about) or have to negotiate with me when the kitchen was available.
What DO the children like to eat? - and when I ask i don't mean favourites but what will they eat without too much fuss? BUT also accept that we ALL have things we don't like/can't eat and that's ok too. My dd hates chips/potatoes generally and chocolate. As well as meat I can't stomach onions/radishes and similar and I have to be VERY careful with spicy food and can't eat citrus fruits.
I think a discussion of not being able to eat favourites every day is worth having too. (I've just had a similar conversation with my 18 year old! She's working full time shifts and I'm too unwell to cook at the moment and she's got herself in a rut, she was bemoaning feeling bloated and tired and I pointed out she'd not eaten ANY fruit for a week! (Plenty fruit available). She's been reaching for quick, easy, favourite foods and forgetting the nutritional stuff I've been drumming into her all her life! I went through with her what she's currently enjoying/not likely to eat when doing the online grocery shop the other day.
I'm a single mum but yes you definitely need dh on board! Not fair him playing the "nice daddy, evil mummy" bollocks! Kids aren't stupid, if they can play you off against each other they will! Practice when the kids ask you something dubious "let me speak to daddy/mummy before I decide"
PLUS maybe remind him this is about his children's health! He is NOT doing THEM any favours if he doesn't help you.