As well as the grip exercises have you tried hanging onto a chinning bar for as long as possible, then pause 20 seconds and get back on and hang. Do that every couple of days with grip work if you like. Make sure you get an exercise free day between sessions, dont want to risk any injuries.
When you deadlift there are options in rep format. You can go continuous, touch and go or rest-pause.
continuous means you control it to a stop at just above ground level and lift back up without setting it down, touch and go means you touch the floor but dont really rest and rest-pause is the same but giving yourself a second or two between each rep, rests the grip slightly - but beware of letting your form go, need to stay tight and not have a rounded back.
There are some proper guides on deadlifting on this site.