Yes, you absolutely need a visa (specifically the K1). Entering the US with the intent to get married without it is immigration fraud!
I've been through this process myself and I can tell you that it's quite the lengthy process and there's financial requirements and many checks along the way. For me it was about 18 months from start of K1 application to issuance (delayed due to COVID) and then about 6 month wait on employment authorization (and greencard about 6 months later).
These times could be much longer if you're not from a western country or mess up the paperwork along the way (easy to do, there's quite a lot of stuff both partners need to file).
Just letting you know so you can start the process early. There's several forums online for people going through such a process with a lot of resources, I'd suggest looking there for other's experiences.
While this is theoretically true, the reality is that thousands (tens of thousands?) of people do it every year successfully. And it's hardly surprising, especially under a pro-immigrant administration like Biden's. I doubt strictly enforcing previous non-immigrant intent in family-based green card applications is anyone's priority.
I'm not advocating people do it. I just think there's a strange dynamic in immigration conversations online where a lot of people talk as if theory and reality are exactly the same. They're not. The reality is that people are constantly engaging in various immigration violations that are overlooked/undeclared/ignored/etc.
You are correct, but if you do it on a tourist visa, you can expect to wait well over a year for employment authorization, which is very difficult on a relationship.
Not to mention you always risk getting rejected (particularly if you posted your intention online) and then being essentially banned from entering the US.
For two grown adults with careers, it makes no sense to risk it, just take one of many legal routes (K-1, work visa, getting married outside of the US and then applying for a greencard, etc).