You haven’t said why there’s stress between your DH and your child so we don’t know how serious or permanent that is.
Otherwise, it gets said on here and it’s often true that the grass is greener where you water it. It sounds to me like there is very little watering going on. It sounds a bit as if you both assume that things would be different/better with a different long-term partner. They almost certainly wouldn’t. You’re both living through the kind of challenges that people in very long-term relationships generally do go through and that can cause us to neglect our couple relationship.
Unless your DCs have special needs, you should be getting quite a lot of time back for yourselves now. Evenings out and dates here and there should be very possible including for 2 working parents. Why are you going to bed at 9.30 pm with the DCs? That sounds like avoidance to me.
It sounds like you’re both choosing to treat family life as all-consuming and your relationship as a problem to be hashed over and solved rather than taking any lowkey pleasure in each other and in your shared life.
You want performative gestures like flowers and for him to make you feel like you’re his world and he’s yours - but that’s romantic moonlight and roses stuff; it’s not how long-term real world love works.
Perhaps you can recalibrate your view of romance. DH and i piss each other off sometimes but we can still find things to talk about, make each other laugh, find each other interesting, enjoy each other’s company and shared activities despite 25+ years at the coalface of earning a living, raising a family and running a household with all the normal day-to-day niggles and irritations. I think that is true romance.
It sounds like there’s a lot of strong foundation here but you are both choosing to chip away at it rather than nurture it. You can’t mend this alone, nor can he, but you can jointly decide to make a bit of time for each other; go out for dinner; go see a film. Water the grass of your relationship.