I went from a lower intermediate (mid N4 grammar, roughly 400-600 kanji) to an upper intermediate (low N2) in about a year so I see no reason you couldn’t reach a N3 level in 6 months.
I would recommend getting into novels or LNs and not just NHK Easy for your reading practice though. In other words, my recommendation is to read the level you’re aiming for, rather than the level you’re currently at so that as you do the grammar lessons, as you learn vocabulary in WK and whatever, your eyes play a scavenger hunt of trying to find them when you’re reading.
This book club is starting soon and is rated to be around N3 level.