Do *NOT* allow him to use flex.

Flex PVC is the worst; after a period of years it begins to sprout pinholes all over. There are probably grades of flex that are OK, but neither you, nor your installer, have any way of finding and determining those grades. And, I know for a fact that much of the flex distributed by the swimming pool wholesale network is NOT an acceptable grade.

Flex polyethylene COULD be OK, but you'd need to use potable water rated line AND fittings. These are NOT what's usually distributed by the pool distribution networks. With poly, the failures are often underground, at the joints

Instead, use pressure rated / potable water rated Schedule 40 PVC pipe (all those ratings should be ON the pipe itself), along with NFS (or Canadian equiv) S-40 rated fittings.

Problems with priming will disappear if your pool guy simply runs the PVC like a plumber, and maintains a steady upward slope from the pool, to the pump pad. That simple, easy, technique will allow the pipe to self-fill from the pool, as soon as you take the lid off the pump. You will need to fill the pipe ditch with a layer of gravel -- NOT sand!! -- so you can create and maintain the grade. If your pool guy says he can't do that, hire a plumber to run the piping -- ANY qualified plumber can do so. (I know: I was a plumber before I was a pool guy; it's how I worked my way through college, and I held a Master Plumber's license for 25+ years!)

"Upgrading" the pump will NOT help with priming issues. What will help is replacing the MaxFlo with the older Super Pump; that is an old design that has the best self-priming characteristics of any pump (Pentair, StaRite, Jandy, Hayward) I've worked with. Installing a 1.5 HP pump on a 12K gallon pool will simply waste electricity for the life of your pool.

On a 12,000 gallon pool, rather than running a single 2" line, you might want to run (2) 1.5" lines, one from the main drain and one from the skimmer, with separate ball valves on each. That will allow you to prime off the main drain, and then gradually cut-in the skimmer line.

This combo will give you excellent water quality and performance:
Hayward Super Pump Max Rated Single Speed @ Amazon
Hayward S244T Pro Series Top-Mount 24-Inch Sand Filter with 1-1/2-Inch Vari-Flo Valve @ Amazon
If you replace the pump with this one, you'll more than save back the additional cost, in reduced electrical costs; on low the pump will move 1/2 the water but take less than 1/4 the electricity. PLUS, filter efficiency will actually INCREASE, resulting in clearer water. (You'll have to backwash on high.)
Hayward Super Max Rated Dual Speed Pool Pump @ Amazon