While TEC have the best level 6 abilities, this is both a strength and a severe liability...as level 6 caps are rare, this strength is often not utilized and certainly not helpful early on (not that TEC need help in the early game)...furthermore, TEC are least able to protect their capital ships...while TEC do have the best repair bay and early access to repair cruisers, late game this is nothing compared to repulsion, shield projection, TK push, jam weapons, phasic trap, and overseers...
Furthermore, while I do agree TEC has (overall) best ultimate cap abilities, I think you are exaggerating their effectiveness...
Finest hour is not even remotely game changing for two reasons: first, raw damage is generally the least useful of any capital ship ability type unless it is on a very large scale (like malice or missile barrage)...though the damage of Finest Hour is debatable, it generally is not regarded as being a huge game changer...second, while finest hour does make the Kol harder to kill, so what? GRG is a near useless ability on fast and faster game speed and does not scale well late into the game regardless of game speed...flak burst, while potentially powerful, in practice is not competitive with TK push or Jam weapons...and if you are making the case that multiple flak bursts can kill SC outright, you are severely overlooking the huge risks and opportunity costs associated with having multiple Kols...
Flux field also is not a game changing ability late game due to hoshiko healing...since this repair ability does not stack, being able to heal more ships doesn't really do you much good due to the nature of how fleets target only a couple ships at a time....overseers and domina subjugators clearly scale better into the late game in terms of healing power as both can heal more health than hoshikos...you could argue that flux field helps spamming dem bots, and that is a very powerful combination, but flux field does very little to help the survivability of capital ships or fleets late game...
Armistice is a great ability but it hardly makes your fleet invincible...since the ability can only be used 1/3 of the time, your enemy has more than enough opportunity to exact losses on your fleet...it would be one thing if TEC had overseers or repulsion, as one of these abilities combined with armistice would make it damn near impossible to kill TEC caps...but, TEC don't have this, and 2 minutes is more than enough time to snipe TEC capital ships or deal some serious damage to a fleet...
Rapid manufacturing is a great ability if used properly and represents the essence of TEC strategy: replacing losses faster than your enemy...obviously, missile barrage is also a very strong ability...
I certainly agree that 4 of the 5 ultimate abilities TEC are very powerful and very useful...however, there are some serious setbacks you are overlooking, mainly the difficulty as TEC in getting a ship to level 6 and then keeping it alive...
Your second point about TEC survivability is debatable...while TEC ships in general are the toughest for their cost, this doesn't magically override the fact that they have less DPS...TEC ships are not all that much worse in terms of DPS, but they also aren't all that much better in terms of survivability when you take into account all the dynamics...Advent shield mitigation, iconus guardians, and shield regeneration on the progenitor make for a very tough nut to crack...also, overseers actually do help keep fleets alive...at first this may seem counterintuitive but if you watch battles with large amounts of overseers, it is not uncommon to see enemy fleets waste minutes trying to kill caps or even heavy cruisers (switching targets works but many people focus fire on caps/SBs before going for the frigates)...
Regardless of that point, fleet survivability is not nearly as important as protecting high priority targets (such as caps or SBs)...you are certainly correct that TEC are more efficient at healing their fleet, but I would say that is not nearly as important as keeping SBs and caps alive...the only thing TEC really have going for them in this regard is repair bays, but those are easy to kill and only apply to defensive battles...
TEC survivability is certainly a very dominant factor early on due to their armor/hull techs and repair cruiser being available earlier...however, late game TEC survivability is easily overrated especially on the offense...Advent and Vasari are much better at protecting themselves from SC, and both have means to keep very important assets (SBs and caps) alive...raw repair rates simply don't scale well into the late game, seriously undermining one of TEC's greatest strengths...
Finally, I find it very hard to argue the TEC SB to be the best, especially for the reasons you have gave...first off, as already mentioned, Advent and Vasari are better at keeping high priority assets alive...while the TEC SB may be the most resilient alone (which isn't even true if you grant a Vasari SB all 4 defense upgrades), Advent and Vasari SBs are far harder to kill...repulsion with TK push and shield regen make Transcencias very difficult to take down...meanwhile, Orkies have the ability to move towards ogrovs and have complete SC cover from phasic trap...to say that TEC SBs are the hardest to kill is simply fallacious...shield restore is nothing compared to shield regeneration (which protects your supporting defense structures as well despite granting slightly less shields) or overseers (which can restore thousands of HP in seconds)...TEC also don't have SC cover (TEC hangars and flak burst are good but don't disrupt bombing runs like TK push, phasic trap, or Jam weapons) and don't have good protection against ogrovs (unlike Orkies ability to move or meteor storm)...
Early and mid game, Vasari SBs are significantly better than TEC SBs...they are easier to deploy, better offensively (since they move), and actually more resilient when alone...late game, Advent SBs are significantly stronger as they have meteor storm and mass disorientation...if you want to make an argument for why TEC SBs are the best, I would not focus on their survivability or the Red button ability (players will simply avoid your SB and you will rarely get the opportunity to use this unless you are fighting the AI)...docking booms is a very good fleet support ability and helps keep structures alive (large formations of gauss platforms are very powerful with this ability supporting them)...also, TEC SBs get more trade revenue and can build frigates, granting them the highest utility in uncolonizable or allied gravity wells where you normally can't build frigate factories or trade ports...