People almost always outright purchase F seats on these flights due to honeymoon or other similar occasion.
Do you have any date in mind? PHX should be a non starter for you. Only 12 seats, twice a day, on a 757 and they are almost always full.
DFW-HNL is more comfortable since 772, 2 flights a day, 36 x 2 lie-flat seats = 72 seats. However, these flights also have 24 premium econ seats each, which is like domestic F -- maybe less comfortable given a longer duration. I'm not sure how it works to use stickers for Y->W on Hawaii flights. The early ~9:15a DFW-HNL flight is probably much easier to upgrade to F, since probably much fewer connecting passengers connecting to such an early flight.
LAX-HNL has 5 flights a day with 16F regular seats on each flight, and the flight time goes by quickly. You could arrive casually the night before, then get to the airport early -- try for the earliest LAX-HNL flight but then SDFC to a later flight that day if F is really that important to you.
The best compromise proposition for comfort is likely connecting through Dallas.
If you were willing to pay the copay, why not consider buying first class outright? You can connect through DFW on a lieflat for < $1k more than an economy class ticket:
https://www.google.com/flights/?f=0#...-20;sc=b;c=DFW
Buy this with an AMEX Platinum and get ~10000 MR points back. In my mind, the added expense is worth it for an 8 hour flight in each direction in a lie-flat seat.