This reminds me of my experience. I've worked for one company based in SEA that had almost identical portals in several countries in the region. Portals were developed by an Australian company and I was hired to maintain existing/develop new portals.
Source code for each portal was stored in a separate Git repository. I've asked the original authors how am I supposed to fix bugs that affect all the portals or develop new functionality for all the portals. The answer was to backport all fixes manually to all copies of the source code.
Then I've asked: isn't it possible to use a single source repository and use feature flags to customize appearance and features of each portals. Original authors said that it is impossible.
In 2-3 months I've merged the code of 4-5 portals into one repository, added feature flags, upgraded the framework version, release went flawlessly, and it was possible to fix a bug simultaneously for all the portals or develop a new functionality available across all the countries where the company operated. It was a huge relief for me as copying bugfixes manually was tedious and error-prone process.