Not sure what your objective is for your original post. If you think Fixer's are the profession of least resistance, I suggest you roll one and try it out. Everything you pointed out is pretty much true, in practice everything more or less balances out.
Here are some observations from playing a fixer:
HoT's are great passive heals (like MP heal pets). The target gets healed at a fixed rate for the duration that the nano(s) are running. But being a doctor is more than just buffing everyone with HP and HoT. The healer's job is to keep one eye on the team's HP bars. If everyone's hurting, pop off a team heal. If the tank or DD is near death, target that person and hit with an enormous single heal. HoT's can only supplement that. They improve the target's survivability, but they don't make a difference if they can't outpace the damage that person is receiving. Yes, Fixer HoT's are very nice, and increase the survivability of the target.
Grid armor looks great until you actually choose to make a grid-armor fixer. They require matter creation, which only the useless item/ammo summoning nanos need, I think. For a class with all blue nano skills, upping the skills to cast a grid armor is a serious pain. It will cost you enough IP at levels below 190 that you will have to think seriously about compromising other skills. Much of my career, I needed MP nano skill buffs and Trader nano point buffs to cast my grid armors. Grid armor is fickle. They poof when they poof, and don't care if you're in the middle of a boss fight. Grid armor is really not an armor at all but a evade and defense buff. At some point, if your mob's attack rating is high enough, their attacks will punch through grid armor like a hot knife through butter. You're invincible for 20 levels, and then suddenly you're running around in a useless ugly blue leotard. Oh, and obtaining a grid armor is insanely expensive and/or difficult.
I already mentioned the all blue nano skills. The fixer's long HoT nano line is level locked, so you can't even get MP buffs to temporarily use a higher one. Most of the fixer nanos cannot be rolled and must be obtained from dyna/boss/chest loot.
Ok, so what's *my* point? Not sure. Being a fixer is periods of the easy life of invincibility punctuated by periods of frustrating farming and getting knocked down and sent to reclaim faster than we can meep there.
That being said, I've not taken a soldier into TL 4/5, so I'm really not the right person to comment. Is a soldier's career consistently difficult from level 1? I think that may well be true. Being one of the top damage dealing professions without any viable self heal is painful. I'm guessing some perks and things may help, but being a profession that is expected to tank without the massive HP, without a viable self heal line, can certainly be frustrating.
But then again soldiers have TMS/AMS, which I've used at low levels. When used correctly, the mirror shields are an amazingly effective tool. My 220/25 soldier org mate rocks and I've watched him solo all but Ian Warr of the mercenaries. I seriously doubt a GA4 fixer would do as well.
Using http://auno.org/ao/stats.php?dimension= ... =0&level=0
as a rough guide, you will see that at low levels there is a slight bias toward enforcers, but at the top of the levels, there is a sudden bias toward doctors and soldiers. You have to ask yourself why that would be if fixer's were indeed better than doctors and soldiers combined.