I still don’t understand why OP doesn’t create his Leap boot USB from 15.5 DVD .iso while booted to 15.1. I just got done booting into the 15.5 installer from an ancient 8G USB stick on a 3.0GHz E8400 Core2Duo HP, which is about 3 or more years older than OP’s Dell. I created the stick logged in as root in 15.5 thus:
# ddrescue --force openSUSE-Leap-15.5-DVD-x86_64-Current.iso /dev/sdg
GNU ddrescue 1.23
Press Ctrl-C to interrupt
ipos: 4413 MB, non-trimmed: 0 B, current rate: 5832 kB/s
opos: 4413 MB, non-scraped: 0 B, average rate: 72351 kB/s
non-tried: 0 B, bad-sector: 0 B, error rate: 0 B/s
rescued: 4413 MB, bad areas: 0, run time: 1m
pct rescued: 100.00%, read errors: 0, remaining time: n/a
time since last successful read: n/a
Finished
I did basically the same thing when 15.1 was the current Leap. Using this method I feel safe in skipping the sha/md5 steps, and don’t need to guess how far along the process has proceeded while using simply dd.