Hey there, I am using the same nutrient blend (MHPgardener)😉
For my indoor lettuce garden I was trying to keep it around 650ppm as well and on occasion will have tip burn as well but could not find a relation to high or low PPM. I tried from 500 to 900 range over the years and did/didn't have tip burn at all levels so I decided to blame the indoor A/C environment.
My favorite lettuce varieties are Buttercrunch, Green Ice, Red Oak, and Sparks Romaine - all from Johnny's Seeds. They seemed to grow the biggest and fastest in my systems. For me Buttercrunch and Romaine got the tip burn most often, the others did not.
Yes, from MHPgardener ;-)
Thanks, will try some you mentioned.