I have found that trying out different sugarpastes has helped. The supermarket ones don't work for me I think because I'm terrible with covering cake with fondant- I take so long and I'm always nervous about it. Renshaws is kind of ok but I always have elephant skin with it! The massa ticino brand has been wonderful to work with- but it is SO expensive. I'm going to try Sugarpaste direct next- it got good reviews from CCers and its much cheaper AND free postage! Also, I've also stuck with the thicker covering- 4mm- as my cakes look smoother.
I have now started smoothing the fondant after it is rolled and is still on the table. After I place it on the cake, I smooth it again straight away. I think its the best time because the fondant needs to be malleable. I put just enough pressure on the cake to get it smooth without denting or misshaping the fondant.
Regarding the crumbs on the sugarpaste- do you crumb coat? This pretty much solves the problem of stray crumbs for me. After I crumb coat all my cakes go back in the fridge then are taken out for the final coat. Also I use a knife to get buttercream onto the spatula that I use to cover a cake- that way if there are any crumbs on the spatula, none of it goes back into the bowl of buttercream.
With the ganache- I agree- practice practice! For the holes and bumps- when the ganache has set, heat up some ganache in the microwave and get it so its very soft but not runny. Cover up any holes with an offset spatula. Just put on enough to cover the holes. Afterwards hot knife all the bumps you find. Watch a ton of videos by different people and take note of their techniques for smoothing ganache. I have found that the smoother the buttercream/ganache is on a cake- the better the fondant looks. You probably know this already but I was an idiot before- I believed that fondant could cover any lumps and bumps on a cake lol!
About the wedding cake- buy a dummy cake and practice like mad. Cover it in clingfilm and practice covering it with fondant. Use Renshaw's for now- not the ones that are already rolled out but the ones you have to knead yourself or invest in better sugarpaste like the sugarpaste direct ones. They sell some in 500g tubs, the rest are in bulk 5kg tubs (these I think are actually cheaper than Renshaws per kilo). Practice smoothing so you get a feel for the right amount of pressure to use.
Sorry for the long post, I just love talking about cakes and I'd like to help others (even though compared to others I'm fairly new to cake decorating so I'm not an expert) like others here have helped me. Good luck x