It is very likely that the PDE5 inhibitors (Sildenafil/Viagra, Tadalafil/Cialis) will help in this situation to generate a better erection, as long as you're not on GTN for angina (in which case they can't be prescribed). You may have some nerve damage from the radiotherapy which is reducing the erection signalling arriving at the penis, and the PDE5 inhibitors amplify the effect of this nerve signalling, and can make up for some lost signalling. Something else you could try is a cock-ring to improve erections - it doesn't need to be very tight. This may also help with the situation above, but may also help of you have venous leak in the penis, which can be a long term side effect of the hormone therapy, although I doubt 6 months HT is long enough to do that.
Go to your GP and ask if you can try a PDE5 inhibitor. Ideally, the GP should also measure your blood pressure, blood sugar levels, cholesterol levels, and Testosterone levels (and might chuck in all the basic blood tests too), just to make sure the diminishing erections are not the result of another condition which needs treating.
A better erection usually makes it easier to reach orgasm. Quality of orgasms is often changed by radiotherapy and prostatectomy, although some people find they get better, but some also report they get worse.
Lack of ejaculate is also a common side effect of many prostate cancer treatments (and all the more common ones). Nothing you can do about this. I actually find it rather convenient.