I heard a lot of spring ping in my 45g FC980C as well. Not particularly loud or anything, but noticeable when listened for. Particularly in the bottom row, as you mention.
One solution presented here is to literally lube the PCB, which I think is more commonly done now. It’s also the method espoused by @anon81470386 in his Topre Modifications Guide (thin layer of Super Lube in his case).
Have you ever tried a “Made in China” FC660C? I have one, and the dome weigh is noticeably heavier than a newer “Made in Japan” FC660C. I would say the domes feel 5-10g heavier, and with more tactility at the beginning of the key press. Further discussion in this Reddit thread, as well as a recent lengthly discussion about dome weigh variability in the Deskeys discord. Another data point for your list, if you ever get to try one out 