I'm not advising you on dosages one way or the other (I don't feel like going there), but as far as you thinking that you will "shrink away to nothing" at TRT doses (150-200 mg test/week), NO, you would not. In fact, based on the current amount of muscle you carry, you would not lose ANY muscle at all (assuming you continue eating and training). Would you lose fullness, some vascularity and hardness? Yes, but steroid induced fullness is just water retention, NOT actual muscle fiber. Steroids, in general, inhibit an enzyme which alters electrolyte balance, causing the muscles to soak up water (think Anadrol or SD). When this happens, the muscle swell up and you may indeed look like you've gained 10 pounds of muscle, but none of that is actual muscle tissue. Some steroids are quite powerful in this regard (Anadrol, SD, M1T), while others are not nearly so (primo, masteron, etc.). This is why we often hear guys say they went on Anadrol and gained 10-15 pounds in 2 weeks, despite already being on high doses of other, less water retaining steroids. Do you think those people actually gained 10-15 pounds of muscle? No. What they gained was intramuscular water, which is why, when they drop the Anadrol, that entire 10-15 initial pounds falls right off.
Even when running a cycle of, say, 500 test and 400 Deca, the intramuscular water retention will still be significant, so if you drop the Deca and lower the test down down to 200 mg/week, you will deflate a little bit and lose most of the other cosmetic effects steroids are known for. This process happens relatively quickly, so many people think they are "losing all their gains" after just a couple weeks. This mentality has caused innumerable bodybuilders to stay on high doses of multiple compounds year year...simply out of fear that they will "shrink up" if they back off a bit.
If you can't emotionally handle the fluctuations in bodyweight, fullness, vascularity or muscle hardness that accompany proper steroid use...and you choose to stay in this sport long term, the chances of you ending up with serious health problem is much greater than it needs to be. On the other hand, if you learn how steroids work and take an emotionally more mature approach by understanding that you cannot always look your best or be at your strongest, you will end going just as far with less harm to your health. In reality, you will likely end up going even farther because you will main better health over the long haul, which is critical for long-term success in bodybuilding. If you do not learn how to approach bodybuilding with emotional maturity, you are doing yourself a disservice. Unfortunately, there are a lot of guys whose lack self-esteem and inner self-confidence prevents them from being able to handle fluctuations in appearance and/or performance.
The truth is that your body, at this point in your development, barley requires any steroids to maintain its current level of muscle tissue. Don't confuse muscle fiber with fullness.
It's the same with strength. One of the ways by which steroids increase strength is through enhanced contractile ability. Some do this better than others. This strength is "artificial" in that it is purely a function of the steroids themselves, NOT due to an increase in muscle fiber. The strength you gain from actual muscle fiber acquisition will stay with you as long as you keep that muscle. The strength gained from steroid-induced contractile enhancement will not. That goes away as soon as you drop the drugs responsible for imparting that effect. For example, if you are using testosterone at 300 mg week and have achieved a certain level of strength...and then you add in some Halotestin, you will immediately add even more strength. If you then drop that compound, your strength will go back down to pre-Halo levels, but your amount of actual muscle fiber won't change. So, when you drop certain compounds and/or significantly lower dosages, you need to anticipate losses in strength, even if the amount of testosterone being utilized if still adequate to main your muscle fiber. The point here is that a loss is strength does not always signify a loss of muscle tissue. This is understand concept you need to understand and accept if you want to progress in the healthiest way possible with your eyes on long-term success.
As a bodybuilder gets bigger and bigger and stronger and stronger, the amount of steroid required to maintain his muscle tissue increases, but the principle above stand true for everyone, regardless of their level of development. You would do well to learn and accept this as soon as possible.