If it were me I would maintain the DC bedroom/s until they are 18, with your DC who is resident the whole time in the biggest child’s bedroom. If the DC are the same sex then you only need one room for them, so 3 rather than 4 bed.
Once they have gone to Uni/college. It depends on how often they intend to stay.
If infrequent/rarely at the same time, would go with the arrangement of having a large two bed (two singles in DC room) with a sofa bed downstairs. Your DC can sofabed it out and let them have use of their room (or share room with same sex child). Your child gives up space or shares room for a night or two being a polite host.
If home a few times a month then you need a spare room, if they are different sexes put a sofa bed downstairs and they can take turns to have use of the spare room. Though they might not come together. You’d have to factor in where they’d study too if they intend to come to you when prepping for exams.
I wouldn’t be in favour of shifting them entirely to a sofa bed at any point as it gives the impression “our child is incredibly important and gets a bed to sleep in, you aren’t really wanted so sleep on the sofa”.
I can understand you wanting to move to a better area for the sake of your DC. And the need to accommodate the SC gets in the way of this goal and you prioritising your child. But, with kindness, you knew what you were getting into, and your DH and yourself can’t negate your responsibility to his children, just because you have gone on to have a child together.