Negative mentions of Thinkific

For your model—fixed cohorts, access keys/invite-only enrollment, capped seats, plus optional private 1:1 upgrades—I’d look at these:

• kajabi.com — probably the closest all-in-one fit. Its cohort courses support scheduled cohorts, paid enrollment, offers, and cohort-specific customers. Kajabi Kajabi It also has separate 1:1 Coaching products and private Backstage client portals, so you can sell a cohort + premium private add-on. Kajabi Kajabi
• maven.com — particularly strong if the cohort itself is the core product. You can create multiple cohorts, hide/show them for enrollment, and use discount codes. It also supports private/unlisted cohorts with direct purchase links and controlled seat quantities. Maven Help Center Maven Help Center You'd likely pair its cohort experience with a separate scheduling/payment workflow for the 1:1 add-on.
• thinkific.com — useful if you want more conventional course infrastructure. Private/hidden products let you control enrollment, and students can be manually enrolled into private products. Thinkific Thinkific Its limitation for your use case is that Thinkific doesn't have a native hard enrollment-limit feature; you'd manage the cap by closing the product once full. Thinkific

How I'd structure yours

Core offer:
Cohort access key → private cohort → X seats → scheduled group curriculum

Upsell:
+ 1:1 add-on → private coaching product/portal → limited number of slots

If you want one system to handle both the cohort and the private upsell, Kajabi is the one I'd investigate first. If the cohort experience itself is the differentiator and you don't mind a separate 1:1 scheduling/payment layer, Maven is worth a close look.

If you tell me your approximate cohort size, price, and whether access keys need to be single-use, I can narrow this down to the specific setup that fits.