With this doing as many sets as you can double overhand before switching to mixed grip…
I found it very beneficial to try and get as many REPS as possible, even if that meant switching grips halfway through a set.
So my first few sets might be all double overhand, and then as my grip tired, I might get 3 reps double overhand, and then 2 with a mixed grip. Maybe the last set was all mixed grip.
And another thing that helps is rest/pause type training - when your grip starts to fail, rest between reps just long enough for it to recover, and do another rep. You can squeeze a few more reps out double overhand this way.