I think Assimil and other series like those are definitely worth it. The problem with Duolingo is gaps in knowledge from a lack of critical thinking and structured learning process. I know a lot of people just tout exposure/reading/watching but at some point you need to write and speak on your own which Duolingo doesn’t help with. Of course getting a grammar book is definitely in the right direction. I feel like I’m midway between A1-A2 and I’ve been studying for just under 2 months. So your timeframe for A2 is plenty. Im not doing anything special, I actually find it amusing when people claim I have a knack for languages. The key is learning tons of new vocabulary every day, studying at least an hour a day and constantly being uncomfortable (lol). I really recommend trying to write as soon as you can form sentences and looking for a Sprachpartner. Writing forces you to think about the words you need (vocabulary) and forces use to truly understand Grammar.