One thing you mention in passing is that your older brother has PC. This is a risk factor for you as some PC is familial. I mention this because it could be a factor in how long you are willing to continue active surveillance. (And yes, multiple mri's and repeated guided biopsies, whether now or after waiting longer, are definitely part of the protocol for active surveillance as opposed to the more passive "watchful waiting" once advocated.)

Since you had 18 cores taken, that was likely 12 systematic plus 6 guided by ultrasound/mri. If you take the same route, your next round would probably also be guided by mri. That mri will tell you if things have progressed. You might be, however, a good candidate for ablation (localized killing) since your cancer appears to be somewhat predictable. I was impressed by, but had too far progressed cancer for, a clinical trial of an ablation approach using magnetic nanoparticles. In addition to radiation, ablation can be done using HIFU--high frequency ultrasound, cryo--freezing, and a few other techniques.

The benefit of ablation is not requiring the risks of surgery, but the downsides usually relate to two aspects--the scarring of the tissue makes reading what's going on more difficult going forward (and it never gets all the cancer) and there is always some damage to healthy cells, either nearby or because the ablation is not tightly enough targeted.

There is of course much work going on to refine the techniques for ablation, as well as a lot of expensive equipment built and marketed for this purpose, so there's a lot of press about ablation techniques.
Let's just say that if any one technique was proven truly superior, some people would make a lot of money and a lot of people would get a better treatment option. I think as someone who already lost my prostate, I can sit back and say that things are not yet that clear.