This is similar to Fairphone’s situation. For the Fairphone 5 they just use an IoT chip with long term support from Qualcomm enabling them to give at least 5 years of feature updates and 8 years of security patches.

Not really a recommendation, but there are some professional displays for the commercial market that can have removable modules for its content. They are more expensive though and they’re more focused at being run 24/7 than looking slim.

Here’s a great video on one that uses a raspberry pi compute module: video