Learning 7K:
I was a 4K-only player like you are now. It took me about a year to get to where I am now - but I wasted quite a lot of time. If you
really wanted, you could achieve the same in 6-7 months.

My only answer to the above would be to
start now.
Oh, and consider learning LNs if you aren't - I don't see a lot of LN scores anywhere on your profile, and if you don't bother with them for 7K, they'll start holding you back tremendously. I was once vehemently against playing those, and it definitely sucked for a while.
LR2 and o2jam are great, but if you don't feel like switching to them, consume as many BMS converts and potentially o2jam converts as well (although personally I don't think the completely-off-the-walls SVs transfer very well to this game).
You have some pretty strong 4K, which will transfer over to 7K better than most people might have you believe. Don't let anyone tell you otherwise.