Your iron storages will all need to be at 18 in order to get to SH20. That will be enough storage for building up to TC20 as well.
As for making pulls, definitely save them for Atlantis. All of the 3* are pretty good to awesome, Wilbur, Proteus and Triton are amazing 4*, and then there’s always the featured heroes, the past Atlantis 5*, and all of the season one heroes.
Plus, as previously mentioned, you’ll get yourself a bonus unfarmable mat for that ten-pull too. The max is 9 chests per Atlantis portal, so you’ll fill one chest per 10 pull (or 10 individual pulls), and your progress will carry over to the next month’s portal [if you don’t fill all nine while it’s open]. So 300 Atlantis coins will put your chest at 3/10 for later.
Good luck in your pulls