interesting question. Tater vines produce flowers which signals the growth of new tubers in the dirt. As for seed, I guess I never looked for any. The vine flowers and dies off, signalling when one can dig for tubers(taters). I get next seasons starts from the dirt pile where I turn my tires over. Always some tiny taters that get missed and left. Who told you the quality deteriorates? Some doofus I guess. They lied. The only thing that has ever effected production and quality here is the amount of water the vines get.
As for reliable crop, well, that revolves around 1-your experience gardening, 2- the proper balance of water, 3- how well you fertilize them, 4- how cruel mother nature is that year. Nothing in gardening is EVER reliable or a sure thing. Way too many variables out of your control. I’ve been feeding myself with a garden for the last 20 years and let me tell ya, it’s feast or famine all the time. There are a few things that help and or make it easier to produce but still not a sure thing. Experience beats anything else.
fyi, I NEVER cut eyes. Every cut is a spot where disease can get to it. I plant my taters whole and discard the big ones for the eating pile. Most of the time, I dont even need to plant a tater, sprouts grow freely in the dirt from last crop, every spring.