I am particularly interested in anything that can help span the gap between The Lost City of Barakus and The Black Monastery. Which is likely a gap between level 5 and 9 or so. Thank you in advance.
Raging Swan Press does adventures with an old school feel that might work for you.
Here's a listing of their adventures here, many of which are also in print on other websites as well- Raging Swan Press Adventures
"Against the Cult of the Bat God" (which I wrote) is for 5th level adventures and pays homage to the original.
"Dark Water Rising" (Ron Lundeen who does adventure work for Paizo) is for 6th level.
"The Sunken Pyramid" (Marc Radle from Kobold Press) is a well-reviewed 7th level adventure.
All adventures have free samples at Raging Swan Press which you can check out.
Check out the Adventure Finder if you're looking for something by level.
You should alter thre thread title maybe?
Well you already know Frog God Games. Maybe Stoneheart Valley or any of their other adventures ?
This thread on FGG forums After Lost City might be of relevance here...
This is the Barakus thread from Necromancer Games where people posted their alterations, errata, maps etc. (but for the 3.5 version)
This thread has an outline combining Barakus with the Vault of Larin Karr and the Grey Citadel.
"Old school" to me by and large means site-based exploration. In AD&D one was always discovering new locals and exploring them in depth whilst putting everything to the sword and taking all of the shiny things.
Paizo's City of Golden Death starts at level 5 and runs to level 7. It is the third in a series so the story works better when led into by the previous installments of the Price of Immortality series. That said, it works just fine with the old "so and so needs/hires you to go do XYZ" method of adventure starting.
Follow that up with the marvelously devious Seven Swords of Sin Kaer Maga based dungeon delve from the 3.5 days. With very little adjustment, it is ready to run for Pathfinder RPG.
For level 8, I recommend another 3.5 module by Wolfgang Baur - Crucible of Chaos featuring a trip to a lost city.
My two cents for what it's worth.
I also support the recommendation for anything by Raging Swan Press.