the 'simplest' fix to at least start integrating the two would be at least having a tab in the ship UI to accept Odyssey missions and on the spaceport terminals to take ship missions.
Greyed out or unavailable according you your current game mode (Odyssey vs Horizons) just like how Horizons content was labelled and separated from non Horizons when i started.
Even better if the mission giver NPCs sometimes offered ship missions too.
That should be an easy QoL implementation that at least starts to put the two in the same fundamental 'space'.
Step two, a (mostly) cosmetic enhancement to build on step 1: new rooms in the stations for the various 'contacts' (authority, blackmarket, interstellar factor, powerplay, material trader, tech broker), on the concourse if possible, new areas accessible by the lifts if not. This would require some new assets, but shouldn't represent a major challenge in terms added code to the game engine.
Step three, a functional improvement beyond steps 1 & 2:
option a) integrate material trading. Let us trade foot stuff for ship stuff and vice versa, and remove the arbitrary trade restrictions between space mats. Balancing the trade values might take a trick or two.
option b) Get rid of the existing mat trader system and let players trade mats between themselves using some kind of offers post/vault system. "Selling 15 military grade alloys, Accepting 30 cat memes".
Option b should in fact be easier to implement since it requires no balance from fdev and what you'll accept for X quatity of Material Y is up to you. It could however represent significant database resources. Also helps reduce a certain amount of grind complaints, you can focus on the mat gathering parts you enjoy more and 'sell' the extra to buy the ones whose game loop bores you.
Obviously there's more (EVA, on foot interior ship salvage, etc, etc) but i think the 3 ideas above at least gets it started going where it needs to go.