Thank you for the update and clarity, it really helps me better answer.
Honestly, I have... quite a number of faction mods. The Xhan upon second look, have lost everything but the capital. After a few hours of playing in the strike mission and invasion logs, the most common are Persean, Diktat, and tri-tach. Legio never even raided them (and got crushed by me). Hegemony and Ludd have been too busy fighting an eternal crusade against my AI using ass. Guest appearances in raids from the Lanestate, ORA, Junk Yard, Imperium, and Shadowyards (that usually fail).
Well the Xhan are a nicely balanced faction and will eventually succumb to the Hivers which is why I separated them in my initial answer. Their benefit to the sector is that they happen to be directly in the invasion path the Hivers use and, unbeknownst to you, they have been sniping them before you even see them. Legio does not invade them it is just that the Hivers struggle when invading them that they spend so many resources doing so, their other invasion efforts are stunted.
As to invading the Hivers, the more militarist a faction the more they will invade them. The amount of factors that effect their success rate are many. For example, if I see four or more factions invading them at once that can sometimes backfire if those factions do not like each other then they are just as likely to fight each other rather than the Hivers.
I don't know anything about starsector modding, how factions work, or how you've coded the faction. But with things like Luna Lib out there, would it be possible to enable the user to adjust settings on the Hiver to give them a little extra boost in a more crowded sector?
It has been a constant struggle of mine to get them to a good balance structure. I do not want them to be the end game big bad, I'll leave that to the Space Doritos. I feel they are currently good if a player has roughly 6 faction mods installed and they will present a difficult challenge mid game. Now I am aware of LunaLib and use it (what a great mod Lucas04 gave to the community) and I have been meaning to get into incorporating it into my mods. Hiver has so many optional files, it would be nice to have them enabled by LunaLib rather then having to delete and rename files manually but I am not sure if that is a function that can be done since that is a matter of manipulating CSV files and I believe LunaLib modifies Java files. But again, I have not delved too deeply into it and it is on my next round of mod adjustments so stay tuned but no promises that my inept coding can handle that level of awesomeness.
(Or turn them into the Tyranids)
Tyranids eh, the Ordo Xenos of the Inquisition has identified your IP and will soon be paying you a visit.
The idea of a hostile faction gradually encroaching, multiplying to more and more planets, and battering down all the factions is something that def changes up the endgame.
If left alone they will eventually become the big bad but that takes several cycles and is not really recommended. In my games The Hivers generally start by taking over Lud (path and church) outposts and colonies then branching out from there. Then the other factions take notice and start sending Invasion fleets to try and take them back for themselves but the planets are now decivilized and not very profitable so it can actually cripple a faction's economy to take back Hiver planets. The Hivers are more of a nasty pest that is difficult to weed out once they take hold. It is best to build up your fleet and have a decent colony of your own so you can afford to lose capitals and piggy back on a friendly factions invasion and provide support. Eventually the sector will take back Hiver infested planets and with your help, wipe their initial systems off the map.
I do take your meaning though that there should be an option to give them more teeth for players who use a ton of faction mods. If LunaLib isn't the path, then I could add a separate optional file that increases their ship stats and fleet strength vs the AI controlled fleets. Either way it has been on my to-do list and will eventually get done, it is just that usually what happens is one of my other mods cries for my attention and squeaky mod gets the update.
But if its a "Lot of work to rework that, you are just using too many factions" situation, that's fine too. Thanks for your work on the mod regardless 
I don't mind the work, I actually enjoy tinkering with the code and learning new things, plus I am retired and have the time. My mods are not done by any stretch but I have been sort of holding off updating them more since the next Star Sector update is impending.
I appreciate the nice word and the Junk Yard shout out
