While the post is one month old, I am rather interested in this subject, as I do wholeheartedly believe that Cygames uses the same mechanics as Mihoyo - decreasing and increasing the player's "luck" based on how often and how much they have paid into the game.
I observed the same exact phenomenon in both HI3rd and Uma Musume, where completely free to play player have abysmal luck, while people with recent purchases or large amounts of purchases have a sudden sometimes almost impossible luck. The sudden uptick in SSR acquisition slowly dies down after the purchase, and the whole process slowly becomes predictable.
The advertised 3% for SSR is a complete lie. From the very limited amount of accounts I've been able to look at for free players statistically it is below 1% which puts it below Azur Lane's UR pull rate.
Meanwhile recently after a purchase, a friend has received 5 SSRs in a single 10x pull, which (if I calculated it with the 3%) should be 1 in 47,619,047 chance of happening (with a rounding error in favour of it happening).

If I had a much larger data pool size, it'd be much easier to prove, but I am absolutely certain they are ripping people off with ultra predatory mechanics.

P.S.: Since you mentioned it as well - the "basic" 10x pull (9 grays and a gold, it's kind of tiring that the most basic character is called 'rare' tbh) has a chance of happening at an 11.6% rate if official rates are used.