Aight, then, that likely gives us the probable founding date and the likely first member: May 2001, SSNTails.SSNTails recently stated on the 20YA server that there was never an SSN-era board.
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Going from the list of members from the EZBoard-era forums, there's a few candidates:
electrokitty (April 2001)
jex (May 2001)
stealth (May 2001)
tails (May 2001) (significant as this could be the banned account that gave birth to the SSNTails name)
thesupersonic (May 2001)
EDIT: For those trying to contact Sonikku right now, he's got a job, y'know. He can be contacted the Discord server as he's somewhat active there.