I'm not sure where you heard the one time dose protocol, but that sounds highly unlikely that it would have any effect on a decent recovery. Also, this is Gonadotropin Releasing Hormone, not HGH. So the effects of the drug are focused on recovery, not muscle maintenance. GnRH by all accounts is something that needs to be administered 4 times a day or so to be maximally effective due to the short half life. I've seen studies where they actually incorporate a device that you "wear" to administer the medication throughout the day rather than having to pin it constantly.
GHRP-6 will be dissapointing for muscle maintenance. Most guys experience good pumps with GHRPs but as far as muscle maintenance when trying to maintain a low BF% I think you will be dissapointed at the lack of action you will see from it.
Ostarine is highly unproven beyond statistical analysis in a laboratory setting. We all know that outside a laboratory setting drugs react differently, especially when considering the vast majority of genotypes people have and what type of proclivity to long term side effects/precursors to disease one may have. The studies I've seen use a dose likely much lower than what a BBer or gear user may require to maintain muscle. So, IMO, I would rule that out as well.
You are basically "cruising" right now with 200mg prop EW. You could continue on that and then start a cycle at some point in the future. However, it is a bit alarming that your levels were in the 700 range with that amount of weekly test intake. At your age it's quite conceivable that your natural test levels (untouched) could be/would be higher than that number. Your E2 levels were normal range at last check so I don't think there's an inhibition of sorts that you need to worry about there. It's more likely that your system is overdue for a complete restart. Your battery is fried and you need a new one, there is no chance of a simple recharge or "jump" if you wanted a somewhat clear analogy.
An aggressive HCG protocol for minimum 2 weeks followed by a heavily frontloaded PCT for a 4 week period may be your best best. You may even want to look into HMG (Human Menopausal Gonadotropin) as a replacement for HCG. It's more expensive, but more effective. It's like the difference between your local strip club and the Bunny Ranch (if you can make that connection, lol).
It's true that you probably won't benefit as much from HGH as someone twice your age, but I don't believe that it will do nothing for you. At a significant dose it will help to preserve the muscle you have gained and will certainly help keep you at a lower BF% more comfortably. I'm NOT advocating that you use HGH because you are still very young, but I just wanted to put everything on the table for you. Because we all know at the end of the day YOU are going to do what YOU want to do. I just want to be sure I've provided you with decent info so you can at least be somewhat informed. Nothing is an absolute truth, but you sure can rig the system in your favor if you play your cards right.