Jayman,
Extensive hairloss to me is the class 6 and 7. Classes 4-5 are in the middle of the road. But it is more comlpex than that. I think there is a tendency for us to categorize without considering the variance within each class. Let me explain. If you take a class 5 with low density, thin caliber, no wave, wide contrast, etc. That guy should not being doing hair transplants.
I was a class 5 in 1996 headed for 6. Finasteride has stabilized my hairloss for the past 11 years. Yet let me humbly say that I have ideal hair characteristics. My terminal hair is dense, coarse, and has wave in it. I have dark brown hair against a fair to olive complexion. Every hair transplant surgeon that I have met tells me I make a great candidate, something I already know.
After seeing virtually thousands of heads over the years, one can clearly pick out the differences within each individual Norwood class. And donor area thinning in family history for example is also a big factor in the equation.
Let me elaborate on your hairline for you and also go back and read it again. I had stated that I felt your hairline is at a mature location where it is right now. So yes "weaker" means further recession as the diffusion continues with time. In other words, diffused hair will eventually lose so much hair shaft diameter "and" pigmentation until they become vellus hair, providing little to no visual coverage and benefit.
Yes I believe your father advanced to where he is because of not having the hairloss meds. Yet most docs would no doubt inform you that you have not been on Avodart long enough to equate it's long term efficiency. Yes it's a risk taken, but at your young age, I would still wait things out, meaning no surgery.
Since there are no class 6, 7s in your history, and remember, you have the maternal genes too, without male pattern baldness right? So like I said, if you want to reinforce your exisiting hairline with some singles, a few doubles here and there behind it, that's terminal hair with virgin caliper, and will help visually. It will reinforce what's there right now "without lowering" anything.
I know countless guys that would absolutely die for the hair you have right now. S.A.F. made a good point in that everyday people are not staring at your hairline. But we tend to be the most critical of our own assessments. You look fine.