The actual difference between (say) SPF 30 and 50 is not a lot, 96.7% UV filtering vs 98% but I’m not 100% sure how that translates to actual rates of cancer.
However the worst offenders in the testing advertised SPF 50 but delivered SPF 4 (~75% AFAICT)
That the duration of protection is independent of SPF makes this particularly true. There are only a handful of places in the world where atmospheric conditions might give a very high SPF marginal benefits.
(This is why the SPF scale is inverted/measures transmittance. SPF 50 sunscreen theoretically allows through 1/50th of the UVB radiation (or whatever wavelengths are specified by your local regulator).)
99% = 1 unit of UV reaching the skin
Thus 98% filtering lets in 2x as much as 99% filtering
Counterintuitively, higher SPF matters a _lot_. The difference is in the _durataion_ of the protection and in the amount of sloppiness you can afford while applying the cream.
Suppose that for you the half-life for the sunscreen is 1 hour. SPF 30 cream would thus decay to SPF 7 in 2 hours, providing little protection. But an SPF 90 cream would still offer quite reasonable SPF 25 protection.
The same applies to sloppiness. SPFs are measured in perfect conditions, with a prescribed amount of the cream spread evenly. So the higher the SPF, the more mistakes you can make while applying it.
https://www.ewg.org/sunscreen/beware-of-benzene-shining-a-li...
Beyond that, no clue.