Ex-raids

The ex-passes use to follow a schedule, but silph road found out and Niantic was like 'hell no you don't know me' and now they're pretty much random.
Two days before my last raid, I got a pass for an hour before my existing raid. I got 8 mewtwo candy that day.

Nope.

There doesnt appear to be much of a benefit of having Gold vs. Silver. Silver is much better than Bronze but the difference between G and was minimal. Based on a Silph Road study.

Best bet is to get silver on a lot of gyms that are ex eligable and to raid them all 1 or 2 times a week.

The community days seem to play a noticeable part whether an EX raid will be scheduled that weekend.

If you know a gym that already gave one or several EX passes, try to do raids there 3 days every week and fingers crossed...

Also, community day helps a lot. Find out which park are people meeting on the community day. Target the eligible gyms in this park 1-2 days before community day and fingers crossed again.

The passes seem to come out randomly, but they come out all at once for any given raid. So, if an EX raid is scheduled for your local park, for example, everyone who is going to get a pass for that raid will get it at the same random time.

The first EX Raid I went on I had a Bronze medal for the Gym, and just barely a Bronze Medal at that. I did 3 raids at that gym (2 Rayquaza and a Tyranitar) in total (likely ever - it's a gym that is difficult to get to in a car, and it's winter right now; if I can't do a raid in my car, I might not be doing the raid) and I still got an Invite. The group of Raiders in my area intentionally targeted that gym to trigger an EX Raid, and about 1/3 of the people who participated got the Invite.

The second EX Raid I went on I had a Silver Medal, had battled for the Gym on multiple occasions, and regularly did Level 1 and 2 raids there Solo. No one expected that gym to be an EX Gym even though it's in a Park because it's not clear if it's in the proper kind of Zone or not (S12 zone? Can't remember off the top of my head), so it was a huge surprise to everyone that we got it.

So what I'm saying is that you probably don't have a problem. It would be BETTER to have that Gold Medal for the gym, naturally, but you've done a raid at the gym - that's like hitting the Legendary Pokémon and getting it into the ball. Now you've got that chance for an EX Invite.