cookie – Multiple websites not working through single checkout – Magento 2.3.5

I have a Magento 2.3.5 installation, succesfully migrated from previous setup. There is a main site with SSL etc that has the entire product catalog and is correctly processing orders. There are 4 additional websites, each with their own domains that have different portions of the product catalog and should process their orders through the main site checkout. Basically, I have 5 websites, 5 stores and 5 store views. This is how everything was configured on Magento 1 and it worked smoothly. This is also what the data migration process migrated to Magento 2 and, except for the checkout issue, it worked right after the data migration, once the webserver was configured correctly for Magento 2.

My issue with the Magento 2 installation is that when I try to checkout from any of the 4 additional sites, I end up at the checkout URL on the main site but with an empty cart. If I navigate back to the originating site, the mini cart still shows the products added.

I have read the documentation, countless articles and posts and still cannot make sense of the situation. I have tried moving the additional websites to be views under the main website. Nothing seems to work.

I have the cookie domain set blank and the cookie path is / for all sites.

My thoughts are that instead of multiple websites, each with a store view, I should have one website with multiple store views (each view on a different domain), although even with this configuration I am still not able to check out through the main site – I keep getting an empty cart on the main site.

I hope someone else has experienced this issue and knows of a fix, I’ve been battling this one for a week or more now.

Thanks in advance.