I think that it is highly likely that von Luck is the longest serving active member based on his March 1999 game report.
Although it’s not conclusive evidence that the user numbers were not associated with length in the club, I did find this profile which like von Luck posted a result in March 1999 yet his user number is 930 which supports the notion that initially at least user numbers were random with the initial data base build.
https://www.theblitz.club/profile/von-sc.../u-930.htm
Update: yeah, I’m user 695 with a start date of 9/99, and user 930 posted results in 3/99; I think that’s conclusive that you can’t go by user numbers, at least not prior to 2000.
But bottom line, the title of oldest is Von Luck’s until proven otherwise