Hopefully it's OK to put this here!! ^^"
There is a series of short interviews from 2025 between Gen Honma & ryo aired in the form of a podcast, in which ryo describes the history behind TAG's formation, and various other details surrounding the band, its music, and UNKNOWN LUCK FES/アノラックフェス (a bi-yearly event for artists from Niigata started by ryo and Kazutoshi).
https://stand.fm/channels/64fc14d181469ca10dcf2774
TAG is said to have unofficially started in 2005!
This is also corroborated in the "What is TAG" section of TAG's official website, which has an overview of the unit's activities from 2005 onward.

Just remembered I can comment. Hi I adore this group so so much. You can get an idea of what they're like by watching the available youtube snippets but aside from that the only way to listen is by purchasing on JP iTunes/Amazon Music or by buying their currently available physical CDs from Zoisite (or secondhand, but they don't show up often at all...) I genuinely do think they are worth all the effort to listen to, their first two singles especially are masterpieces.