Absolutely. For applications I built myself, I would often just roll the possible configs for us as files in the container, and you pick one with a single env

Absolutely. For applications I built myself, I would often just roll the possible configs for us as files in the container, and you pick one with a single env var (realistically this was like "dev", "beta", "prod"). Not great, but meant I could avoid the clusterfuck that is specifying all my AWS configuration in env vars on a per deploy basis. ConfigMaps could sort of help with this, but I found what you found - having to base64 a config file as a field in a ConfigMap and make sure it gets mounted correctly just seems pretty baroque for configuring something with a file. And at the time, ConfigMap changes didn't even cause a redeploy, so your pods would for their lifetime keep on using a stale config!