Radioman is simply for fleet related things like joining leaving creating declaring on harbors etc. Don't really see why it needs to do anymore.

how about at level 100 or so (or make the level vary by perk, and to get the perk you can class the BO to get it), you can give the BO a perk. Make it a one-time-only perk, make it unchangeable (Don't sell a premium item to change it, either, make it 100% final). Some examples:

-"Experienced Lookouts" where you are notified when the enemy can see you. If your ship cannot see a submerged submarine, and the enemy team can only see you because a submarine is providing vision, then you are not notified, and if you are only a shadow on the horizon, you are not notified either.
-"Veteran Helmsman" where your ship's turning force is increased
-"Experienced Spotters" where if you can see enemy shells incoming, you can see where they will land in advance. If the ship's speed is fast enough then this can be adjusted by the devs to decrease the warning time so that all ships will recieve equal chance to dodge the shell, discounting the player's skill and reaction time.
-"A Solid Team" - This perk does not come into effect until level 110 or 115, the caps for your crew serving under that BO increases a bit, but not too much, and SD is still capped at 900.

Although I cringe to think of what the actual implementation of this would look like. 

1.) Restorers are designed for helping low level players achieve the SD cap earlierand CVs with improperly classed pilots achieve 900 SD.There is no possible way to make restorers more possible. It would require editing every single sailor in the system and is definitely not worth the risk.

2) Radiomen are required to Join a Fleet, They are required to create/operate a squad or fleet. They are required to declare harbor assaults.

Quite honestly makes it one of the most important sailors in the game to have.

3.) This is called the weapon caps.
Gunners have the same thing, this is intended. Its part of the balancing mechanics.
The caps you experiance, are also on all surface weapons. Completely intended as I already stated.

4.) You are not actually exceeding the sonar limit (As to my understanding there is none) You end up seeing the BO's maximum visual searching range. You cannot see anything in sonar that is outside the BO's sight range as the BO's sight range is the maximum sell spotting distance for everything with your ship of any kind.

5.) Bridge Operators, are again, a very important sailor. It:
Generates medium SD and Repair rate
Sets ship sight range.
Sets weapon guidelines and spotting distance.
Obviously required for filling a ship tree. BOs store ship tree completion.
BOs keep ranking data.
lvl 120 BO is required for HOF rankings.
Sets torpedo spotting distance.
Sets plane spotting distance (For bombing warning and plane proximity warning)