It's a difficult market to really judge.
There are definitely collectors out there, but it's not a huge market compared to sports cards.
I sold his base Upper Deck Yong Gun rookie for $1500 two years ago, and the market was clearly defined because they exchange hands often.
I would use eBay sold listings to really get a gauge of the value and using comparable player (maybe more Mario than Gretzky).
I'm not sure how much an autograph would bump up the value, but if it were to be autographed, I would reslab it and have it authenticated by PSA.
The best time to sell would honestly be both of those times - retirement / HOF - and not "today". Anytime he's front and center, interest would pick up.