Radical suggestion:
To solve "Low Tech ship non viablility" problem, I suggest remove them from the game entirely.
No ships - no problem.
And now, when I have your attention, lets get serious:
Comrades, problem is not exist! 10 pages of thread for nothing. Low-tech ships does their job well. They are crude, cheap (in price), spendable hummer to smash enemies in a brutal massacre of numerous berserk warriors. That is their job. If you want to anal-dominate the entire sector with no casualties, using unfair natural advantages, and then proud yourself, how awesome you are - that is for High-tech ships. And the midline silently laugh from them both, able massive spam and bites deadly at the same time.
If you feel like low-tech not satisfying your demand - change them for something that match. Or change the demands! Do not try to embroider with hummer!
Do some ships need a slight nerf/buff? Well, yes, off course. But this is like a witch hunt now - let's nerf/buff entire class based on hull pain color! Because I like/dislike their appearance, and want firepower match that.
P.S. about fuel consumption and speed: have anyone ever seen a dozer? That is what low-tech ships are. I've rather add them more hull and armor (or even type-based damage resist), than improve fuel consumption or speed. Same thing to maintenance: have anyone ever tried do repair a dozer? It is much expensive compared to your sedan!