I have reached current endgame, with 600k lorentz factor and lvl 93 antimatter engine mass & auto research. I have ~600k antimatter engines.
I have 103684m2 of fuel net, and L41 antimatter engine consumption. Fuel gain scales really fast with speed, and with any more net I will start gaining fuel faster than I can use them. I will eventually get too much fuel that weigh my ship down.
Looks like fuel net has 'air resistance', so the terminal speed depends on how effcient you use the fuel. Strangely, the fuel effciency is boosted by the number of engines.
3 suggestions:
1. Combustion engines are useless. I would better skip them, because they can't gain any meaningful speed to get lorentz factor significantly above 1. The game only starts when I get to fusion.
From lore perspective, combustion engines are useless if you want to get anywhere near c.
I think you should directly remove combustion engine tech, and start the player with fusion, and maybe add 'exotic' after antimatter.
2. I think fuel should have a limit, so fuel doesn't weigh the ship down.
3. I think it would be interesting to add some way to gain more building materials.