I just did mine via the THIM app. Passport scan didn't work, but was quick and easy otherwise.
Curious as to what you actually did with the new THIM App.
It aims to modernise the arrival card process which is mandatory for all foreigners entering Thailand.
Since 1st May 2025 travelers to Thailand have been required to complete the TDAC, 72 hours before arrival.
The TDAC web based system is still available, but the THIM essentially replaces the TDAC within a mobile application.
Immigration plans to roll out additional services such as extensions and 90 day reports, but these are not yet complete and part of the later roll out and expansion of the system.
You have to complete what I'll refer to as the 'management' page, which is where you can scan and download your passport biodata page, followed by your personal information, and the health declaration.
Were you booking an arrival in Thailand?