I don't know about increasing the hull. I mean, yes it's kinda the most obvious choice I guess, but it's so straightforward and I think it would make it more like "meh now it's just another ship with decent armour/hull" feeling. I like Spire because it's glass cannon, it's high risk/high reward(arguable since it's easy countered) kinda thing. It's challenging and fun to use it. So I would like to see it stay glass cannon and I would add something else that would make Spire shine at.
Here are some of my silly ideas, I am never good at giving ideas about balancing, but here we go... Maybe someone can make something of it and improve on it...
1.) Make reversing a lot faster. Maybe that could be a Spire thing. Spire and Mobula both have good firepower and front facing guns. Mobula can dodge vertically, let the Spire "dodge" horizontally, but maybe only in reverse, leaving acceleration forward the same. In addition turning speed could also be increased, so Spire could outrun other ships with reversing back and turning fast and losing the opponent behind obstacles/walls (canyon ambush for example).
2.) Make armor on the sides stronger. So maybe Spire could have reinforced sides, so it would take maybe 50% less damage when hit to sides? This could for some tactical dodging. Fire guns, turn ship to the side while reloading guns, so you will take less damage from weapon fire and ramming. If you want to take risk and keep firing, you would be weak, like now, so it would require good communication and teamwork to decide when it's good to use sides to "tank" or when it's better to take a risk and keep shooting. Back of the Spire would stay the same, so flanking would still be possible on spire.
3.) Buffed Armor on Spire could recieve 75% instead of 30% increase?
4.) Maybe make it somewhat that when Spire would use tar, that something would lead/push tar in front, so Spire would always leave tar in front of itself, thus making opponents think twice when charging or for medium/longe range for losing spots?