I think your math is a bit off. Glory keys have a 1:10,000 chance of a mythic and gem keys have a 1:1000 chance. Not sure where you are getting 5000/500 from?
1500 glory keys gives (1 - 0.0001)^1500 chance of not getting the mythic. So about 14% chance of pulling it. You’d need to open around 7k glory keys to have a 50:50 chance of getting the mythic.
This is generally correct only if the actual individual pRNG rolls are independent, which requires a proof. Some previous tests may indicate that this is not the case, and individual rolls are not random and independent. However, considering the law of large numbers, which may be applicable to a sufficiently large number of rolls, the assumption of randomness and independence is probably correct, thus supporting your estimation. One has to keep in mind that individual pRNG rolls are broken into batches of 200 for convenience and obtaining an extensive strict proof of randomness and independence is virtually impossible in this case by design of the developers. It does not have to be done on purpose to obscure the actual numbers, just a means to avoid server overload and achieve some convenience for the users who might otherwise burn through all their keys without gaining actual benefit.
“Only” took me about 18.6k guild seals & 4k glory wings this time…
Also the first one I get in a 3 month dry streak on new mythic week, after using a total of about 36k guild seals + 200k glory wings + whatever many gem keys (and even some vip keys).