One of the best fixes you can give any of these astromechs is to give them a secondary effect equal to R2-D6.
All useless mechs, take away the action header, make it at certain situations like said (end phase, whatnot), and also state that if placing this astromech onto a ship that is PS3+, the ship gains an EPT slot. IMHO, This immediately brings X-Wings, and even E-Wings, right back into top tier. By also doing this, you aren't really boosting Biggs either, because Biggs wants r4d6. Here's how I would do it:
R2-D6, 1pt - Gain an elite slot if PS3+ and don't already have a slot. Reduce the cost of Elite upgrades by 1 to a minimum of 0. Can only equip to ships that don't have Elite upgrades. Keep Unique
R3, 2 pt - Gain an Elite pilot skill slot if the ship doesn't have one and is PS3+. When attacking with your primary weapon, if you spend a focus token to modify at least one focus result during the "Modify Attack Dice" step, then assign one evade token to your ship. (Some randomness still there, but at least you are not sacrificing offense for defense. Even if it only stops one damage per match it is better than Hull Upgrade.)
R5-K6, 2pt - Gain an Elite pilot skill slot if the ship doesn't have one and is PS3+. After spending your target lock, you may immediately acquire a target lock on the same ship. You cannot spend this target lock during this attack. (By taking out the randomness it is better than it was but still strictly worse than Fire Control System because you need a lock to spend to set it up. That is just how bad it is.)
R7, 2pt - Gain an Elite pilot skill slot if the ship doesn't have one and is PS3+. Keep normal ability. No change needed.
Targeting Astromech, 2 pt - Gain an Elite pilot skill slot if the ship doesn't have one and is PS3+. Keep normal ability. This encourages the use of Stay on Target.
R2-F2, 3 pt - When defending, if the attacker is not in your firing arc, roll one additional defense die. (Wasting the action for one die is almost always worse unless you know you will be defending at least 4 times. Its supposed to be Biggs droid and he was getting shot at from behind by Vader so let's put some theme into it.) (This one stays as you designed, I like it)
R5-D8, 3 pt - Gain an Elite pilot skill slot if the ship doesn't have one and is PS3+. At the end of the Combat phase, you may discard a facedown damage card, then roll a defense die. On a Focus result, discard one of your facedown damage cards. (At 3 points, it was still way too tough to use. Make it so just on focus results, and can always happen. Combined with IA, it can prove to be very tanky, as long as it doesn't blow itself up in the process, and only works on facedown damage cards. Porkins will still love this mech).
This does two things. It fixes useless mechs, and it also fixes X-Wings and E-Wings. This doesn't affect T-70's that much, they like all the other mechs. It might affect ARC's, but that's not necessarily a bad thing. Beside, the Arc's we have tend to want R2D2, BB-8, and R3A2, which those should all stay as is.