The reason for producing things at a lower TL, is that lower TL stuff is cheaper. Not just in higher TL systems but intrinsically. With an effective trade network this enables low TL worlds to contribute to trade and thus balance their trade.
In a colonial fleet I don't need TL15 jump 6 ships as I am not defending a sector, I am defending the subsector at most and probably systems a few parsecs away so TL12 Jump 3 is plenty. Higher TL power plants are more expensive but if you need to move vast distances will be able to offset the extra cost of high TL in savings in overall size and thus fuel. The bulky lower TL power plants also don't matter in ships if the overall power requirement is lower. I don't even need jump drives at all for my system defence vessels.
Higher TL weapons and armour are always going to be useful, but I can also afford to have more layers of cheaper armour as it doesn't need to be moved as far. Similarly weapon systems.
The colonial forces need to be largely funded from local taxation. Every Cr saved is a Cr you can gift to your Billionaire supporters in tax breaks