It's probably formulated ideally for what it's intended. Are you
having problems with it cracking/failing?

Making up your own concrete in a mixer and vibrating it in situ would
probably considerably strengthen it, but I wouldn't bother unless it
was necessary.

The advice I had from the fencing company was "dig your holes as deep
as possible, and aim to have a big lump of concrete down the bottom".

Worked fine for my 2m tall fences in an extremely windswept fenland
location - still perfectly vertical several years later.