The percentiles are a bit misleading these days - if you remove test takers from one particular country (which has unusually high Q scores) from the percentile calculation, your Q percentile right now will be in the 90s, so your scores are very balanced. Your mathematical foundation is so strong already that you'll want to avoid generalized prep materials, which are typically aimed at people with much lower scores than yours. The way a Q34 level test taker should prepare is very different from how a Q48 level one should. Instead you should work with material specifically intended for high-level test takers. In particular, avoid any materials that emphasize 'strategy' over content - what companies call 'strategy' can be useful for low-level test takers, but it's useless or actively counterproductive on higher level GMAT questions. Instead you should be trying to develop your conceptual knowledge, so study from materials that emphasize content, that actually teach how to think about math in the best way.
Your highest priority, for practice questions, should always be official questions, and there are many good official resources available, so get all that you can - get the latest OG book, for example, and the official Question Pack, and all of the official practice tests. If you want additional hard questions, you'll want to be sure to use questions that emulate the style of real high-level questions. Some prep company questions are 'hard' only because they're long or complicated, and real GMAT questions aren't like that. Bunuel at gmatclub has designed a lot of high quality high-level questions that he has posted here, so that's one very cheap resource you can use. My materials are also all intended for high-level test takers specifically, so if you might be interested in those, send me an email. I wouldn't recommend taking any kind of course, since group classes won't be useful to someone at your level, but if you want to save time, you'll progress most quickly with a good private tutor. But tutoring in the GMAT market is very expensive, and given how well you did without it, you should be able to reach a Q50 or Q51 only through self-study, if you study from good resources. Good luck!