Injectable steroids are injected into muscle tissue, not into the veins. They are slowly released from the muscles into the rest of the body, and may be detectable for months after last use. Injectable steroids can be oil-based or water-based. Injectable anabolic steroids which are oil-based have longer half-life than water-based steroids. Both steroid types have much longer half-lives than oral anabolic steroids. And this is proving to be a drawback for injectables as they have high probability of being detected in drug screening since their clearance times tend to be longer than orals. Athletes resolve this problem by using injectable testosterone early in the cycle then switch to orals when approaching the end of the cycle and drug testing is imminent.
Testosterone can be administered parenterally , but it has more irregular prolonged absorption time and greater activity in muscle in enanthate , undecanoate , or cypionate ester form. These derivatives are hydrolyzed to release free testosterone at the site of injection; absorption rate (and thus injection schedule) varies among different esters, but medical injections are normally done anywhere between semi-weekly to once every 12 weeks. A more frequent schedule may be desirable in order to maintain a more constant level of hormone in the system.  Injectable steroids are typically administered into the muscle, not into the vein, to avoid sudden changes in the amount of the drug in the bloodstream. In addition, because estered testosterone is dissolved in oil, intravenous injection has the potential to cause a dangerous embolism (clot) in the bloodstream.
You seem to know where you’re at. I’m about your age, I think, with about the same attitudes too. One thing I did is swap out benching for the standing military press. It seems to have been a good move. My shoulders are far less tweaky and weird and it doesn’t seem like I’m missing anything by retiring as a bench presser. Your post mentions a ‘lack of spotters’ and that’s one of my favorite things about no longer benching. The danger/stress is gone. I absolutely blew out my shoulder once while benching and after that it now seems sooo possible that it could happen again. I just don’t bench anymore and it’s not the huge gap that I know it would seem like it is. If you’re tweaky and in any way discouraged with your benching career, you might want to consider it. Hit the standing military press hard. Learn it from Rippetoe youtube vids. It’s more a comprehensive lift than I ever thought it was.