Really glad my comments were not taken offensively.
I do not think DOT 19" tires for the front are hard to find since so many ADV bikes use them like the Tenere, BMW GS/GSA, and the VeeStroms. Maybe in a true knobbie you may well be correct.
The forks/shock travel differential is not too hard to adjust. I am not familiar with your specific forks, but most forks have a spacer set up that can be moved around top to bottom or bottom to top to adjust ride height and or travel.
The 21" wheel size is not an issue at all. If that is what you want....run it man. To change to a 19" is not hard either or as expensive as some might think, just for future reference if you should choose to change it. Spokes are about $80 and a so is a wheel ring. You can lace it yourself. Not super cheap, but not out of reach if you chose to go that route.
Good on you for the build though. Something that was your dads and you can keep along time is a cool thing. I am sure he will love seeing the build, and seeing you enjoy it. I would if I had a son.
it is easier to ask for forgiveness than permission