Keep in mind that the game continually gives new content (some permanent and some time limited) every month. Doing everything that comes out in the month can net you about 60,000 to 80,000 gems. This only counts what’s new every month going forward. it doesn’t count the current permanent content you have access to. Without strong teams you may not have access to everything, but most isn’t behind tough content. Saving a stash for that banner is good, but it wouldn’t need to be all your gems. Also keep in mind that Yshtola is part of the dimensions end order Ex shop. So if you eventually got those tokens you could save those and when you draw on that banner, if by some terrible luck you didn’t get her Ex while going after her LD/ BT you could purchase it there. As far as saving for the BT and burst remember it’s 75,000 to guarantee and LD and 125,000 to guarantee a burst. I think you have a lot of time to save up for her. Her burst is estimated for April 2021 if we continue following JP release pattern.