If your GM let's you homebrew the Solarian I would play that. If not I would play Soldier.
What’s more interesting for you, being great in combat, or being able to manipulate/negotiate outside of combat?
Assuming you value all classes equally, it sounds like the group could really use a face -- the Lashunta Envoy in my group isn't great at combat, but is invaluable outside of it.
I'm playing a Soldier 1/Envoy 2, so far it's been great fun both in and out of combat. I don't do as much damage as the melee soldier, but I can certainly hold my own. :)
What do you mean by "front line combat type"? Someone who specializes in melee and literally goes on the front line, or someone who can do a lot of damage?
I think someone's done the math, and Envoys can do more damage than anyone, they just do much of it vicariously through other characters, by increasing their hit chances. If your only concern is how much damage you personally do, the soldier is the gold standard.
My experience reading and playing and running AP/Professional type adventures has always been that you need well rounded characters to fully participate in the adventure. As a group, you can certainly each specialize in one aspect of gameplay and easily complete the adventure, but there are going to be parts of it where you're literally twiddling your thumbs and watching the other characters.
So, without spoiling anything, I would make sure that whatever character I built, I would have something to do in single target combat, in group combat, in single target social encounters, in encounters with technology, in puzzle/knowledge/trap type encounters, and in starship combat. Those seem to be the majority of the types of encounters/challenges the game was designed for, and I would expect to experience most or all of them in every AP.
The Envoy is pretty innately the most well-rounded (maybe tied with Operative) class. Depending on skill selection, they can pretty much fill any of the starship combat roles. They're the only ones (I think) that can do in combat stamina healing. They're pretty much the best at social encounters (maybe tied with Operative/Mystic). Since "trapfinding" no longer exists as a thing, they can also potentially do much of the corpse role, and if they pick the right skills, also do much of the computer work (at least as a backup). They have access to all but a couple of the knowledge skills, and theiving abilities. About the only place they lack is overland travel on new planets (no survival or science skills).
The solider is less limited than the Fighters of old, but they're still essentially combat specialists. Unless you spend feat slots, or specifically built for it (like Ace Pilot Dex/Ranged Soldier), you're probably going to be the gunner in starship combat. They don't have any of the social skills (except Intimidate) and usually would want to dump Charisma, so they're pretty much standing in a corner looking menacing in any social encounter. They don't have any of the trapfinding/breaking skills and lack most of the knowledge skills so they're usually not very helpful in those encounters either. They do have survival, though, so they can be a provider during overland travel.
My recommendation would be to go with Envoy, and pick skills based one what everyone else does to fill in all the gaps first, but that's just me. I personally like Halflings more than Lashuntas, but that's just a flavor thing on my part (I think a halfling envoy ship's captain would be fun as heck).
I am running Dead Suns now, and I can say that both envoys and soldiers contribute greatly in the AP. However, for the love of all that is Holy, make sure at least one or two people are trained in the Culture skill... I don't consider that a spoiler, just something that will enhance everyone's enjoyment of the AP.