To address one question that most of us probably just regard as a given:
If you install /e/OS yourself (or with the help of a friend or whomever), it is free, you do not buy it. However, if you are happy with it, you might want to consider donating to those who provide and maintain /e/OS.
And the above is most likely equally true for other custom ROMs like CalyxOS, IodéOS, LineageOS etc. as well.