Is Bitcoin PoW actually SHA256 + Merkle generation? Or have I misunderstood coinbase/append? Miners can mutate nonce (32 bits) + time (mutates once a second). This allows for 232 (~4 billion) hashes per second. That's not enough anymore for our ASICs as they perform in the TH/s now rather than GH/s. So we allowed miners to mutate the coinbase transaction, but this requires us to generate a new Merkle tree. This means that a miner needs to generate a new Merkle tree every 232 hashes. at 1 TH/s The miner must generate a new Merkle tree 250 times per second.

TLDR: Is Bitcoin PoW actually SHA256 + Merkle tree generation? And not pure SHA256?

If I'm correct in asserting that Bitcoin PoW is SHA256 + Merkle tree, does this slow the commoditization of ASICs and therefore slow decentralization, as ASICs now must be more complex than if they did with just SHA256 + nonce mutations?
You are correct that effectively Bitcoin PoW involves computing the Merkle root every now and then in addition to the hash grinding.

However, this is negligable. Even ignoring nTime rolling, the Merkke root computation is just a dozen or so hashes every 232. It's so little because not the entire Merkle tree needs recomputation; just the coinbase transaction is modified, along with n Merkle nodes above it (assuming up to 2n transactions).

As far as I know, the burden of Merkle root computation is so low that it is generally done in the miner controller rather than in the ASIC itself.