The last time I used Coinbase Commerce did not have the option to accept USDT it was not listed on their checkout page. I don't know if they added USDT but they do support other stablecoins like USDC and take note of Coinbase-managed and self-managed accounts if you want to have full control of your wallet create a self-managed account it supports auto-convert but only for USDC, unlike their Coinbase-managed account.
There is a lot better out there than Coinbase Commerce - If you are not from the US, I doubt you'd want to use that service when there are other open source options out there, or developers who might build just as good of a solution so that it can stay in your control.
Do you need only USDT or any other coin/tokens? What is your volume?
For Bitcoin, there is
https://btcpayserver.org/ open source and seems what you are looking for, but we don't have an alternative for Ethereum unfortunately.
Take a look at
https://cryptomus.com/ as well.
Cryptomus looks well designed, doesn't look open source though. I will give credit that it looks to accept quite a range of different instruments. Have you used this before personally?
Has anyone else tested this or used this practically?
This looks some kind of marketing strategy. Ask a question and respond to same question with a website link. Anyways, it is a great thing to see a website which can help deploy a payment gateway .
Which one is the marketing post?
If you are looking for an open source option, the best you will get is ---->
https://btcpayserver.org/ and then automate the conversion with each transaction or once every day/week to convert Bitcoin to USDT, or use closed source options in which the fees are either based on a percentage of each transaction or monthly subscriptions.
In general, if you still insist on the open source option, there is ---->
https://github.com/zixipay/zixipay.github.io that will provide you with what you are looking for, but I did not find enough reviews for the code
Out of curiosity, has anyone tried to re-create btcpayserver for other alts on Bitcoin's codebase? (I know it would be difficult for non-btc codebase, like evm based)