I generally agree with the sentiment here that assaults are kind of the least of your worries. However, don't feel like you have to rush them. For most normal people, Alexandrite will always be the limiting step. Even if you struggle with Assaults now, you'll still probably have them done before you finish Alex. Getting Captain and starting the quest isn't going to help you finish that any faster.

Just make sure you are always doing Salvage and making money to buy more Alex and also doing Einherjar whenever you can. Everything will sort itself out. But I highly recommend you either get some friends or at least one dual-box account to make things easier.