First, I do thank you for your kind suggestions, and I'll answer one by one.

Q1.
You mentioned about the 'additive' defensive repeatable techs, which can be ovelapped with existing multiplicative one, and can be applied together with damage AND fire rate RTs. But I won't apply it for several reasons.
1. That makes too many RTs.
2. Removing fire rate RTs is not only to solve the imbalance of the offense/defense RTs, but also to make fire rate bonuses more important and valuable. In Stellaris, fire rate bonus is used commonly as a 'elite' concept, like in Clone empire or Millitant ethics. Those effects are severely blurred as the fire rate RTs are researched.
3. Fire rate RTs also distort late game combat by twisting the PD/strikecraft/missile balance. Remember that RTs can be accumulated indefinitly.
4. And that change is also very mod-incompatible, cause technically, there is no way to use ship 'size' in the game, and I have to mention all the ship size names - e.g. Cruiser (Vanilla), Strike Cruiser (NSC), Dreadnaught (RS) etc... - which is very frustrating as a modder.
For these reasons, I will not keep the fire rate RTs and/or add new defensive RTs.

Q2.
As you mentioned, hull is critical cause it directly involved in combat power such as fire rate. So I tried to take a more conservative approach to strengthening it. If 2% is really too weak as a single level RT effect, I'd fully agree to your opinion and change that number to 5% as you mentioned. I'd love to hear more from other users on this issue.

Q3.
No. As of now, I have no intention of doing that. That needs too much ideas and works. And I like 'vanilla-ish' Stellaris experience.

I wish this may be a good answer for your questions.
Thanks again for your concern and comment.

Good answer on the Fire Rate! While I've always enjoyed more RT so it doesn't feel like I'm spamming the same ones repeatedly when playing a game well past its prime, you make a very good point that it's both more useful than damage and a rarer bonus overall. I'll probably use this mod if I ever try PvP games with friends again.

I would still suggest Hull getting a 5% tech, but considering the rest of your answers and the current balance so far, I wouldn't suggest any other changes in the meantime. Hope your mod gets some more recognition!