Brachytherapy, external beam radiotherapy and surgery all have more or less identical outcomes in terms of 5 years progression free, 10 year survival, % needing salvage treatment. The idea that EBRT or brachy are more risky because they can't be followed by salvage surgery is a bit of a red herring as, although failed surgery can be followed up by salvage RT, the outcomes are not great (statistically).
With a G6 I can see why you would hesitate over radical treatment; if my husband had been suitable, we would have gone with brachy although this would have been with hormone treatment. As it turned out, his surgery failed anyway.
Have you explored possibilities such as focal laser therapy, cryotherapy or HIFU? You may have to go overseas to get FLA or green light therapy but the side effects are significantly more tolerable.