And this is what I use, 18"x 6" blade gives a perfect hole for 4" posts, about £30 and a good 15 years old, digs hundreds of holes every year. Simply grovel on knees and grab out spoil by hand. For more than 30 holes, hire a 2 man motorised post borer!
I've been doing fences for 30+ years. I use a graft.
https://www.e-hardware.co.uk/6080090-sitemate-ultimate-fencers-graft.html?gclid=EAIaIQobChMI5v31gbra4wIVBrDtCh1cygHwEAQYAyABEgKDv_D_BwE
I bought mine from my local fencing place. They're not cheap but they last forever and hard as fuck.