I've been playing for about the same amount of time and was in a similar position and felt I wanted to stretch myself a bit and also get more out of playing the bass.
I bought an upright a year or so ago and that spurred me to get some lessons primarily to show me how to hold the thing, a few weeks ago I restarted the lessons again with the intention of learning some theory and I have to admit it's been really enjoyable.
I've bought a ton of books Chuck Rainey, Ron Carter, Ray Brown, the Simandl book just arrived (woohoo) which are great but I think having a teacher has been invaluable, it gives you the impetus to practice cos you don't want to look like a fanny and it costs cash. Also it's it's quite inspiring to hear someone who can really play but is sufficiently different from your approach that you are really learning. I suppose it might be different on the electric where I have some skills and I might be a bit more resistant to being taught
It's opened up a whole new range of possibilities and is great fun, as well as making Amazon happy
Oh and I've been looking at the Major's Bootcamp Sessions which are great, I mean what a lot of effort to make that fantastic resource available and the numerous transcriptions on basschat.