Hi,
I just came back to the game after years of being away and fortunately have some cartel coins to spend. I bought three crates, but only 2 of them had platinum items. In the years since last playing this game I have played quite a few different games with similar monetization, and pretty much all of them, barring Battlefront 2, had a guaranteed drop of the rarest item type in purchases like this.
Some of it is based on a simple count, like a lot of gacha games. Some are based on a secondary currency in each pack that gives access to purchase any of the items available in the game. Very few let simple statistics rule the day.
(On a side note, most gacha games also have to post the drop rates for their items, which I can't find seem to find anywhere for SWTOR. It is kind of scary that Las Vegas slot machines are held to a higher standard than Star Wars is. It also says something when a large percentage of scammy mobile games are more honest with their customers than Star Wars is.)
My suggestion is just to include a guarantee that every 30th box has a boosted 100% of containing a platinum item.