Hair Bonding Treatment - Which One?!

My hair has never been thick and lush, but in the past couple of years I've noticed some pretty bad breakage and thinning. I had to cut in a fringe to disguise where two chunks had just broken off, still not sure why those happened but possibly from heat styling or something that I didn't notice until later. I have un-bleached hair, no chemical treatments, dyes, perms, etc. Any damage is mechanical. The ends look so wispy, and I've always has sort of fine hair anyway, and even having a good few inches off at the hairdresser a few months back hasn't solved the problem, it just got wispy again as soon as it started growing out. So I figured I would try a bond repair treatment to see if it fixed the problem.

My issue is, there are SO many of them, and the "good" ones are all pretty expensive with about 10 products in their line, so trial and error is going to get very expensive and frustrating. So I am here to ask the experts!

I've heard of two popular ones, Olaplex and K-18. I did see a video by a guy called the blowout professor where he said to just use the main product (bond repair treatment) and not to bother with the bond repair shampoos, oils, etc because they're money grabs.

Anyway, thanks in advance!