We already have a system for "research patents." It's called the Patent Term Restoration Program [1] and it allows the FDA to grant 5 year extensions for patents on approved drugs, up to 14 years of exclusivity from the date the marketing application is approved.
Giving pharmaceutical companies the full 20 years after approval, regardless of how long it took, would create a perverse incentive to patent early just to protect existing patent marketshare. It would also be extremely unfair to everyone else like the aviation/aerospace, semiconductor, and energy industries where it can easily take 5-10 years to get from patent to product and cost just as much. It's much better to subsidize research (which the public does on a massive scale already) and drive down the regulatory burden of approval, which we can do without decreasing safety by some targeted reforms of the FDA (mostly by giving it more power to organize clinical trial infrastructure and enforce transparency and trial integrity). Giving pharma more power is not the answer.
[1] https://www.fda.gov/Drugs/DevelopmentApprovalProcess/SmallBu...