...and multiple hash (>100000) to slow down brute-force, this will be really reliable, I believe. 
I don't think that will be effective in the long run. An Nvidia V100 can theoretically do 10x as many hashes as a K520 and they were released just 3 years apart. So I imagine if not the current Ampere cards or next year's cards, one of these years someone's going to be able to pass 100K hashes/s at the same speed people pass 2000 hashes/s (number of PBKDF2 rounds of seed phrase) today.