I can empathize with the OP. Years ago, when materia was still expensive, I went through 161 Strength Materia IV on the last slot of an accessory for my tank. Melds that defy the odds do happen and it stinks.

With that said, I don't think they need to change the system. This is the nature of randomness. Melding DoWMHL gear is entirely optional. There are guaranteed slots. When it comes to DoW/M gear - in some cases you can obtain gear better than gear obtained via running dungeons (drops/tomes) by overmelding. There should be a price for that capability. Bear in mind, that price is a lot less than it used to be. Materia comes from many different sources, not just spiritbonding where you lose the piece of gear. For DoH specifically, the cost to meld is very low in comparison to what it used to be. There was a time when you had to meld 4+ slots of every piece in order to just hit the thresholds to even attempt the craft. Those days are gone. Sure, if you want to use the meta-macro to craft, then you'll pentameld. But again, that's entirely optional.

I tracked my materia melding in Google Sheets. I used to sell pentamelded gear on the MB. I had a lot of data points. Remember that 161 attempts at a 5% meld? 14 attempts @ 5% should yield a success 50% of the time. That time it went way over. But over the course of thousands of meld attempts on gear - that 161 was evened out.

In other words, I find the RNG they use to be pretty good over the long term. We always remember the 161 fails. But how many times do we have a 5 or 7% meld succeed on try #3? How many times does a 17% meld succeed on try #1? It all evens out.