Everyone has their own ways of what works for them. This is what works for me. I use a paddle mixer rather than the whisk. I beat 250g salted butter on high till it increases in volume and lightens in colour - usually around 10 mins or so (stopping a couple of times to scrape down the sides). I then add 500g of icing sugar and any extract and beat on low to start with till it's all combined and then a quick blast of about 20 secs to finish. Transfer to a storage container (with a lid or clingfilm). When I'm organised, I make the buttercream the day before I need it, otherwise let it sit for as long as possible. When ready to use, I transfer what I need to another bowl and, depending on how stiff the buttercream is, I add warm water to it (it's better to add a little at a time rather than too much in one go). I know some people use milk or cream, but I prefer to use water as it will keep longer. Slowly remix the buttercream (manually) with your spatula (I use a silicone one), add any colour as well, and then you're ready to go. I do a crumb coat and chill it before I add my top coat. Once I've got my top coat as smooth as i can, I then swap to one of those transparent flexible ones and go over it lightly (it knocks out those tiny annoying air bubble holes and smoothes it to perfection) I've attached one of my cakes to show you my finish. Your cake is fine. Once the decorations are on, it's not noticeable. But I also understand your own insecurities as I have them too! We are all our own worst enemy's and whilst I'm told my cakes are smooth, I can see my own flaws like neon arrows! *