Yes, the Ko'olau & Pono homepage is seriously outdated both in technology and content. I like their products very much though, having owned several of their ukes in all sizes and lines, so here is what I know about them:
Pro Classic is their top line with wood binding, gloss finish, an upgraded rosette, and option of a slotted headstock. Series 5 indicates that it has additional purfling, so the difference is purely cosmetic, adding more "bling". Purfling and rosette were made of Abalone until 2017, then they started using different materials due to changes in regulation. Mostly, they have been using Turquise stone for the purfling and rosette on Series 5 models since then.
The Master Series was a limited edition run in 2017 which took the Pro Classic line to the next level by adding a side sound port, a beveled armrest and a silky satin finished neck, along with an understated look (simple "P" logo instead of "Pono" on the headstock, no dots on the fretboard, but still markers on the side of the fretboard). The top wood for this line was hand selected by Ko'olau luthier Noa Bonk, either Cedar or Torrefied Spruce. As mentioned, the Master Series was a limited edition run and hasn't been available again since then. I don't know if they are planning to make such a run again, but I sure hope so.
Since then, some of the features introduced with the Master Series have been used on a few Pro Classic models as well. All of them now have the silky smooth neck added to the gloss body, some have a side sound port, and some have had the simple "P" logo. There have been a lot of different combinations of these features lately, blurring the lines between Pro Classic, Series 5, and the Master Series. But to me, it is one of the strengths from Pono that they make so many different models with different features: with or without a sound port, with or without added purfling, with a slotted or a solid headstock, with different wood combinations, etc. Out of all these possible options, I'd just pick what speaks to me (or ask them specifically about a model that has all of the features you want).
Again, this is what I gathered over the years of following the brand and its development more or less closely. I'm not associated with them in any way, I just like their products very much. I hope there is some helpful information in there for you.