I think the core trashing component of the card is very good. I'd remove the protection from attacks, cost it at $4 instead of $5, and call it a day. I would guess that's a fun and balanced card, but of course it would need playtesting.
I would say the protection from attacks is simply too good on boards with strong attacking strategies available, while being worthless in the absence of attacks. It looks like a $6 or $7 card to me (for its power level on attacking boards), but then nobody would ever, ever buy it on other boards. Even at $5 it's a terrible card if you're not using it to defend against attacks: it's simply too slow at trashing to wait until turn 5-7 to play it.
A couple minor technical notes, even though it's already clear what you mean for the card to do:
1. This isn't a Reaction card, just an Action-Duration. Think about Lighthouse, which isn't a Reaction either. The defining feature of a Reaction card is that you can reveal it from your hand in response to some event.
2. You'd need to reword it a bit for consistency with existing rules and card text. I'd suggest: "At the start of your action phase, you may trash a card from your hand. If you do not, trash this card. While this is in play, when another player plays an Attack card, it doesn't affect you." The last sentence is taken straight off of Lighthouse.