thac0 was not the same as the current system but trea𒁏ting AC the opposite. AC was opposite, but each class had it's own table to determ💎ine what it had to roll (including bonuses from magic weapons) at what level to hit someone with an armor class of zero (and then you'd add the armor class to the roll). I don't recall the progression, but it might get easier approximately every two levels for a fighter to hit someone, every three levels for rogues and clerics and every 4 levels for mages).

The artic𒆙le also conflates advanced dungeons and dragons (which is called 1e) with the original ruleset (called original d&d or OD&D). OD&D used race as class, and this was continued in Basic D&D (basic was a separate "streamlined" set of rules that the owners of D&D released concurrently with AD&D and maybe 2e).

If you write an open world for your players and they find something they want to d𓆉o, let them d꧙o that. You've admitted you don't have any other "A Story" for them to follow.

You're not a computer game where everything is written before the adventure starts and they have to sta💦y within your preconceived bounds.

Write them an adventure that's level appropriate and ties into spider folk. If they're not level ready to encounter actual spider folk, let them encounter their human or elven or tieꦿfling (whatever is appropriate for your world) allies that start them down that path.