Hi,
in order to get the rigidity you want, you need square rails and bearing loaded cars. Hiwin do a nice range, you need the heavy duty design but 15mm rails will be fine.
There are plenty of Chinese made copies, often a lot cheaper, but of lesser quality. They would still be a big improvement on your 'spaghetti' round rails.
If you want quality without the cost maybe you could try second hand. I made my mini-mill from second hand Bosch-Rexroth rails and was very happy. My new mill
is much bigger and I used new-old stock heavy duty 20mm THK rails and cars for a fraction of new price. I also used second hand, near new ground C5 32mm double
nut BNFN THK ballscrews, they were a steal at $1000 for all three including shipping. Had I bought new they would have cost $3000 each plus shipping!
https://www.ebay.com/str/industrialp....m47492.l74602
Craig