Whether or not you are slacking all depends on how fast you want to learn the language and progress. Saying that, I’m currently studying Japanese language and I probably put most my study time into learning kanji at the moment, but during my first year it was probably more time put into grammar, once you grasp the basic grammar you start to retain other grammar faster and easier etc, thus why in my second year of university I now spend more time on Kanji than grammar.
long story short, I would say you should also split your time and learn grammar, but it is ultimately up to you where you want to spend most your time and it also depends on your planned schedule to whether or not you are slacking.
