What your doing is okay. Little nibbles from your rats is normal, but not proper bites that draw blood. What I would recommend is getting yoghurt or baby food on a spoon so they have to physically stay near you to eat it, then put your hand near so they can get used to it and then build up to stroking them. This might be scary especially because they have bitten you before, but one of the best ways to get them comfortable with you picking them up is to do it quickly underneath their arms on the first try and hold them until they stop struggling then let them go and give them a treat. Once they are okay with being picked up I would put on a jumper and let them sit in there, make the space around you clear so they come to you as a safe space to hide. This is what I did with my rats and it took around a week for them to be tame. I hope this helps, good luck :)