The LUKS2 source code mentions there is an attack on Argon2 which they prevent by setting rounds to 2 or 4 iirc. Not sure what that's about though, would need to look it up again but perhaps heed the recommended settings to avoid such pitfalls
As for parallelism=1, why would you do that? My understanding is that you want it to a point where it'll saturate your memory bus because then you're getting the most out of your system -> making it the hardest for the attacker. (Pretty sure this takes more than one thread on a modern system because I've tested that argon2id with the same number of rounds and memory gets faster with more threads.) Beyond that, increasing memory usage is good because that dominates the die space of an ASIC which in turn dominates the cost, if I remember and understood the argon2 paper correctly. The repetitions value is the last thing to touch, namely when you've run out of threads and usable RAM (such as when it gets into DoS territory)
(I focus on argon2 because that's what I've read up on. Scrypt is also a good option but I'm not sure the same advice applies because it has this TMTO attack. The underlying concepts will be the same of course, but tweaking one parameter before another may play into the attacker's hand.)
