~snip~
When you say "Even the smallest USB ASIC miner will be thousands of times better than a great GPU, so you can't realistically mine Bitcoin with a GPU", could you suggest a model and approximate price? I am asking this because I have been mining BTC using Nicehash since last year with 8x 3080s!
You haven't been mining BTC using Nicehash with GPUs.
You have been lending your GPU altcoin hashrate to someone else, and they're paying you in Bitcoin. That's how Nicehash works.
To actually mine Bitcoin, you need to do sha-256 calculations. ASICs are way better than GPUs for that.
A Compac F, which is a small USB ASIC does about 300GH/s, and it sells for about $200-300.
An Apollo BTC, which is a somewhat larger yet still silent, perfect for home, does about 3+TH/s, and it sells for about $500-600.
A GPU is not really comparable because it will be usually hashing altcoins, which have different difficulties. But for SHA-256, they're way worse than any of those ASICs.