I wanna give my 2 cents. As far as I know from the current state of the game, Volga will be involved with each love interest at least to some extent storywise. Thus, IMHO, he can romance any one of them, in a way that it is plausible, from the story point of view at least. I'd be curious to know which is considered canon by the devs, or whether any romance option will have an effect on the end game.
I do agree with OP that Aoba is more like a mentee to Volga, who could help Aoba be more self-confident. It is not necessarily romantic. Romancing Aoba might be pushing an otherwise wholesome mentor-mentee, strong friendship too far IMHO. I also agree that Tharsix would work better as platonic, though I can see them work. Volga could help Tharsix come out of his shell. It could be interesting and lead to believable romance.
I think that Kakula and Cyrus are similar that they are very open about their interest in Volga. I think Volga would like to slowly build it up, being this pushy might not be the best for him, methinks. I don't think Lambert is for him either, seems a bit sly and dishonest, and Volga would prefer a genuine relationship. I can totally see Volga with Aquilo, Darius and Maximus though. Especially Maximus. Both being soldiers, I can see it as a common ground the romance could build upon. All three characters seem to be honest, loyal, strong and protective and could resonate well with Volga.
But, fortunately, you can choose, or you will be able to choose any one of them.
Maybe the end game romance is the King? :D Would that be Volga's wish? :P