First of all, I don't understand the meaning of I would have more agency in career. Not only do I not know what agency in career actually means, but it's odd in terms of syntax (I think it should be either my career or the plural careers). Moreover, in context, it sounds like a good thing rather than a bad thing. If it's something that's good, then not having waited for it to happen would not actually be great.
Also, I'm assuming that year is a typo, and it should be years.
That aside, everything after the first comma is talking about the future, not about the past.
Discounting that part of the sentence, there are two main parts to consider:
I think it's great I got to come . . .
This describes a past event. (Although it could be an event that is in the immediate past, which would put it in the syntactic past, but contextual present.)
. . . before 10 or 20 or 30 years had passed.
This describe a hypothetical future.
There is nothing wrong with mixing tenses in certain constructions:
I believe that I will have some ice cream.
I know now that I should have turned left instead of right.
It's quite ordinary to talk about a hypothetical future in the past tense (or a mix of tenses):
If I hadn't taken that job, I might have been homeless years from now.
If I hadn't taken [past tense] that job,
I might have been [past tense]
homeless years from now.
In this sentence, the assumption is that I will still own the mansion in the future. (Of course, nothing about the future is actually known.)
You can't replace before with after, because the meaning of the sentence is that not even 10 years have yet passed. So, everything after the first comma discusses a future event.
If you want to change the sentence to put it all in the past, other syntactical pieces would also have to be changed:
I think it is great I got to come, you know, after  30 years had passed.
I think it is great that I only came, you know, after  30 years have passed.
(It's great for some reason. But because the stated reason in the original sentence doesn't make sense, I have left it out.)
The first sentence is almost how you phrased it, except that it has 10 or 20 or removed. (As does the second sentence.) Why? Because it's in the past. It's already known exactly how many years passed. It's not technically wrong to include such a large date range—but it's unusual to the point of it not normally being said. (Perhaps somebody with amnesia or other memory problems would make such a statement.)
The first sentence would be used if the getting to come happened a certain length of time prior to the sentence being uttered; the second sentence would be used if the getting to come happened in the immediate past.
While still describing a hypothetical future, it's also possible to only replace had with have, while keeping before:
I think it is great I got to come, you know, before 10 or 20 or 30 years have passed.
Or you can use a future-tense auxiliary verb in combination with the past tense:
I think it is great I got to come, you know, before 10 or 20 or 30 years will have passed.
Or you can leave out the auxiliary verb altogether:
I think it is great I got to come, you know, before 10 or 20 or 30 years passed.
I think it is great I got to come, you know, before 10 or 20 or 30 years pass.
One version of tenses may sound more or less natural to certain people, but there's nothing ungrammatical about any of them.