We have a similar dynamic. Just getting my husband to commit was like pulling teeth. He would do everything last minute.
I just plan everything now. But I mostly like to plan and have realized that even if he did it, most likely I wouldn't be super happy with how he did it. I make the plans and tell him where I will be going and does he want to come along? i.e. I stopped waiting for him to agree to it. He 100% of the time has come and had fun and is a good travel partner. It just seemed like he struggled making any decision and that drove me crazy.

Who makes the money?

I'm like this for home repairs and home maintenance. It drives my DH crazy. But I'd just let things go until it was really, really, really necessary. I bet eventually he'd plan a vacation but it might be years and who wants to deny themselves those years of trips and fun. Is there ANYTHING you can offload to him and try and balance things?

My DH is like yours so (despite me being the primary breadwinner and working longer hours) I just essentially research and plan the vacations alone/decide on what car, etc we are going to purchase and nominally run the decisions by him.

I do often feel slightly resentful about it, but the alternative is we either just don’t travel/make major purchases, or I spend hours browbeating him to “help” with the planning process which results in him booking poorly researched and overpriced inferior options.

Have you tried chat gpt? Mine has planned some great vacations

Some people like planning vacations and some people don't.

How do (or did) the conversations go when you did discuss these things?

People have different styles. My DH would figure out what car to buy after researching for an hour. For vacation he’d just hop online and book air and hotel in an hour after making some quick comparisons. From his end, there isn’t a lot to discuss about either of those things and he sees no point in having a lengthy discussion about it. I have learned to work with this and have my preferences in order and things already narrowed down before we buy/book. And I plan vacation details myself (excursions and reservations etc) because that is where his style comes up short (if left to him, things would book out).

My DH doesn't like talking about vacation planning or buying large purchases (cars).

I'm viewing these things through the framework of mental wellbeing, family needs, fun lifestyle and school schedules.

If yours is like mine, how do you solve this? He's just not a partner at all in the conversations or he doesn't really care.

It's a lot of work for me too to have to think about all this bc I also work and do a lot of the domestic work load.