Quick question about Illium star chart merchant.

You can only go to Illium pre-Horizon if you have modded your game. In a non-modded game, Illium is unlocked after Horizon.

As for scanning and mining: I recommend only mining Good and Rich worlds. Anything below Good is not worth it in my opinion since that adds unnecessary amounts of time not playing the more fun parts of the game. With Good and rich worlds, you should be able to get all the resources you need before Horizon.

Of course, if you are modding your game, you can give yourself enough resources from the start without needing to mine. So there's that. You can only really
mod with a PC copy of the game. At one point you could mod the Xbox version of the game (I was able to for a few specific playthroughs) but I do not know if you still can or not.

Whenever I play ME2 (I play on Xbox), right after recruiting Mordin and researching the scanner upgrade from Miranda, I typically spend several hours scanning planets. Sticking to Good and Rich planets is enough to get the resources you need before Horizon, including enough Platinum for the Med Bay upgrade. And even if you do not or get tired of scanning, a few planets post-Horizon will be enough.