Just answering your questions in any random order:
(a) Most people find that most double baskets are easier to use than most singles.
(b) If I were you, I would aim for a shot that would extract 30mL in 30 seconds, give or take a few, but I would stop the shot a little bit early ... so, for example, it might be 20mL in 22 seconds or so (or 40mL from the double basket). Sorry to complicate things, I just always felt with my old machine, which was the same sort of machine, that getting a full 30mL (or 60mL from a double basket) was a big ask. This way, you get the more chocolatey part of the pour and dont risk getting anything that might be a bit sour in it.
(c) The single hardest thing - and probably the most overlooked - is dosing. Youd be surprised how much difference dosing can make. If you dose, say, half a gram more or less, that can be the difference between gush and choke. Most hard copy literature doesnt really emphasize that, which is sad. You need to develop a consistent dosing technique. There are heaps and heaps of different techniques that work. Personally, Id just grind until the portafilter was heaped up, rap it on the bench a few times to settle the pile down, then get something with a straight edge, like a knife, and use it to level off, fillling any gaps. Then tamp. If you do something like that, you get the dose relatively under control and you can start focussing on grind coarseness.
(d) When youre sure that youre dosing consistently, you might want to look at getting a grinder with stepless adjustment. I have found that most grinders with stepped adjustment dont have small enough steps. Failing that, you might want to get a burr replacement - apparently they wear fairly quickly in your grinder. Before spending money, though, you should test your consistency by getting out the old stopwatch and shotglasses and seeing if you can get the same volume within a few seconds time five times in a row.
Hope that helps - if I missed anything important, Im sure someone else will fill in.