I visited him twice (May and December 2021) with ESTA. We applied for a K1 visa after my first visit to him.
The first visit was at the height of Covid-19, so there were a handful of foreign travellers at the immigration. When I told an immigration officer at Detroit Metropolitan Wayne County Airport about a visit to my boyfriend/would-be fiancé who lived in NYC, the officer immediately asked me to come to the office. They asked me many questions about our relationship, and then one of the officers asked for my phone. They checked my WhatsApp chat with him. The officer called and asked him the same questions they had me. After they inspected my two suitcases and backpack, they finally gave me an "admitted" stamp on my passport. I had been there for more than one hour.
The second visit was more manageable. I brought our copied NOA 1 (I-797C), our attorney's letter, the letter from my workplace (university) and the bank statement to the immigration. The attorney told me if I showed the immigration my strong ties, which meant my job and money in the bank, with Japan (so I had to get back to), they wouldn’t deny my entry. However, I was asked (again) to come to the office where my all documents were scrutinised by the officer. After 30 or 40 minutes later I was free from the office.