When optimizing for long-term maintenance, we have a few choices. I like to bet on monorepo. A monolithic repository is a simple idea. You organize the code of all your services in a single repository. It has a few advantages over using a separate repository for each service.

Reusing code is easy. Once you abstract a coherent unit of code into a module, you can then import it from anywhere.

Continuous integration runs tests against the entire monorepo, so once PR is merged you bump the version of all sub-services and there is no doubt what versions are compatible with each other. Version 1.2 of service A is always compatible with version 1.2 of service B. This is why complex projects with multiple dependencies often use monorepo as well (Babel, React, Angular, Jest). Due to the very same reason, large-scale refactorings are also feasible.

You maintain one third-party dependencies tree. It's too easy, especially with all NPM goodies, to end up with two different versions of the same library and having to sync them across different repositories manually gives me a headache. Having one main package-lock.json is a real time-saver.

Monorepo forces collaboration, it encourages having the same coding style by having a single config for your linter/code formatter/module bundler and so on.