With 5000+ emails stored from over several years I have not found an easy way of deleting all emails over 6 months old as an example.
I use the iPad mainly for an aviation application and for email on a trip. When it's switched on after a period of no use I have to wait while it catches up on the missing month or so of email, with no good junk mail filter like my PC at home.
I just went back and revisited the post I had replied to - you said you either have to erase one email at a time, or all emails - not selecting blocks - in that case, I was correct.
You then changed the argument to filters/spam/and years-old mail. A tablet of any kind probably isn’t the best device to make up for years of poor maintenance of email.