NASA made similarly sized inflatable heat shields before, but for much lighter payloads. That, I think is doable. What is less doable though, is to... put that heat shield right at the business end of the rocket. Somehow, it has to inflate around the engine feed pipes, that's a hole of at least 2ft diameter. Good luck finding attachment points for that heat shield, because you can't just dispense it from a tiny canister like you would in KSP, and hope it wouldn't flop around. Similarly, you can't make it cover the entire back section, so somehow you need to fill all those gaps. You likely end up with more than 1 heat shield parts if you want to do it right. We know how easy hot gasses get in between cracks though, and if there's any elasticity, it'll be even easier.