Signing in here for the first time. I did this same cross about 20 years ago, when the golds were first available. Took a few years as it did for you, but eventually I had hundreds of pure breeding gold longfins. I sold some on Aquabid and also to locals in the fish clubs. I found out a couple of things that might interest you:
Parents seem to eat the gold babies much faster than the normals. Found this to be true in short fins as well. I got best results in tanks choked with riccia, or else in planted tanks where I pulled the parents out when the first babies were sighted. The normal long fins would allow a significant number of babies to mature with them.
There are 2 subspecies of white clouds, both were considered extinct in the wild 20 years ago, not sure if more have been found in the wild. The northern strain vanished first, and was replaced in the trade with the southern strain because it tolerated warm water (Florida fish farm ponds) better and also it had white/yellow tipping on the fins. The strains obviously interbreed, but if you find original photos from the wild, you can see that the northern strain had fins that were white or clear close to the body, with bright red toward the tips. The southern strain was the opposite, red near the body with white or yellow edging on the tips. I had both fin color variants in my gold longfins and rather liked that, but the ones that took after the northern strain had much more total red area on their fins and were real shop stoppers.
My gold longfins seem to have accumulated some of the worse of both (highly inbred) strains. They really liked the unheated tanks in my unheated basement, where the temps varied between 60 and 72. But most everyone else that tried to breed them failed to even keep them alive. My water was also very soft and slightly acidic, maybe they liked that too. All I know is I could raise them by the hundreds, but others often could not. I once had a 55 with lots of live plants (riccia, java moss, java fern). There were easily hundreds in that tank of all sizes, all the time. I would just net out some to take to auctions and you could not tell I had removed any.
I think some of their descendants might still be out there, I see them advertised on Aquabid every now and then. I really hope they did survive, I liked them.