There are several avenues that could we worth exploring:
Reconsider Your Overall Goal
You said, "my overall goal is to be a well-rounded knowledgeable person...". To be honest, I'm not persuaded that this is a worthwhile goal - or even a meaningful one. The truth is, nobody is ever really "well-rounded". We all have our areas of knowledge and areas of ignorance.
Personally, I choose to embrace that imbalance rather than to fight it. I do this by playing to my strengths and enjoying becoming more knowledgeable in those areas. At the same time, I leave other areas for people who are gifted in those areas. When I need to know something about ballet or football (not my areas at all) I ask someone else. If other people need to know about LEGO, they ask me.
BTW, I'm not saying you should never read books on subjects outside your areas of expertise - if you did that, you'd never develop other interests. Rather, I'm suggesting you read books that really interest you, not just because you think they will make you "well rounded".
Choose More Suitable Books
If it takes 1-2 months to read a book because that's how long it takes to let the ideas sink in then perhaps the books you're reading are at too high a level for you? Can you find introductory texts on the same subject? That way, you'll have the foundations in place when you tackle the more advanced texts.
Prioritise Your Reading
Given how long it takes to read a single book, it is really important that you choose books that give you the most "bang for buck" - by which I mean, will give you maximum understanding about the most important topics in the least amount of time. I've written an article on choosing books that you might find helpful.
One possibility is to focus your reading on the things that you can put in to practice. The things you learn from books are are negligible compared to the things you'll learn from actually doing things.
Or, you could just pick books that are both exceptionally well written and extremely interesting.
Speed Read Anyway
Another approach is to speed read and risk missing important stuff - but not to worry about it. I suggest that you might learn more overall by speed-reading two books than slowly reading one book. Of course, only you can make that call, and I totally respect you if you carry on reading the way you do just because that's your preference. But if you're reading to learn, consider skimming and skipping bits that are less important.
Read Books in Parallel
Your own suggestion - reading more books at the same time - has certain merits. However, rather than debate these, why not just give it a go? Whilst other people have all sorts of experiences that could usefully inform your decision, nobody else's experience of reading will ever be the same as yours. The only way you can know for sure is to experiment. And I can understand your reluctance - why waste valuable reading time on an experiment like this? However, when you consider the potential benefits, I think you'll agree that the risk is worth it. So give it a go and see what happens!
Just Be Slow
Finally, you could just accept that you're slow. Being a slow reader isn't necessarily a bad thing. And being a reader is an awesome thing.