Describe the feature you have in mind.

Edited because I thought of a much more simple way to describe what I want

In the 1.18.2 version of the mod, the Jetpack behaves strangely when combined with elytra. I think that it would be best to make them work like this when combined.

Pressing the Jetpack Engine toggle will swap between the 3 states of having Just the Elytra, Just the Jetpack, or Both active.
While both the Elytra and Jetpack are active, Holding space while gliding activates a version of the firework rocket effect as long a space is held. Instead of being a single boost of set power and duration per use, it is a continuous boost that adds speed for as long as the button is held.

I do not know if this is how it works in the 1.19.2 version already, but at the very least I think this way of simplifying the button usage and switching which effects are active (instead of having the up arrow key be involved) is better.

Does this solve a particular problem?

Yes, It makes the jetpack more intuitive and useful to use with elytra.

Currently the jetpack behaves like this:

• When in flying position, the only thing it does is boost you when you press space (should also be a continuous like you described)
• When upright, it behaves like a normal jetpack would do, for example hover if hover mode is on. You can of course not boost yourself up when you are already in the air, because this would also activate the elytra and move you into the flying position.

The only missing thing from your description, would be that you are able to use the jetpack normally, while wearing the elytra by being able to "disable" the elytra ("Just the Jetpack"), but don't think this makes a lot of sense, since you cannot de-activate the elytra in any other setup and you can just take it off.

I just checked and the functionality works in 1.18.2 exactly as I described it above. The boost is not entirely continuous, but you can only notice that when you are almost completely still (right after starting to fly)