lounge_welcome.body is the content for the welcome topic inside the Lounge category, as far as I can tell editing it has no effect. Same with lounge_welcome.title, vip_category_name and vip_category_description, I think they’re just seeds for creating the Lounge category.
There isn’t an explicit message sent to users when they reach TL3 but they are granted a badge and a notification is created for that, so badges.regular.long_description might be the closest to what you’re looking for.
An advantage to my mind is that users can reach and never (unless manually done by admin) lose TL2, so it’s a one-way street of engaging with the community to some degree (which can of course be adjusted by admin) and then gaining access to additional resources.
I’ve been less active here on meta recently so I’ve been bumped back down to TL2, losing access to Lounge. The user experience of gaining access to Lounge and then losing it, including my own posts as if they don’t exist, felt kind of weird to me.
An area for the most active users of course has its place in some communities but the default welcome topic doesn’t really emphasise that continued access is incumbent on continued levels of activity, which is probably part of why it felt weird despite knowing when and why I would lose access.
I don’t think there is any magic with the Lounge category (there is with Staff) but the welcome topic does have useful information about what a TL3 user can do and it doesn’t really cost anything to leave it there.
If you don’t think TL3 will be useful for your community, you can disable it and avoid some users briefly obtaining it by setting tl3 requires days visited to 101, i.e. tl3 time period + 1.