Interlay uses Intel SGX? Although not “decentralized” per se, yes you could’ve a much cheaper BTC bridge that ways.
You could have several signer SGX enclaves, each held by exactly one trusted “invulnerable” collator. A small threshold of these sign the BTC tx, which includes some validator bitfield and a commitment, and imposes some waiting period. Anyone could dispute the BTC tx by fronting the 0.005 BTC fee, after which the singers must post the full tx with all the validator signatures. All this still involves some development work though. Is that worth it vs ploding along and more slowly building a decentralized BTC bridge?