Unfortunately the biggest "mistake" is architectural - your systems seem to require an outage to ensure code and migrations are in place before bringing the system back online. This may not be the fault of anyone - legacy systems are what they are and may never support in-place updates.
Ideally your migration code sits in the same repo as your code, so the same version will deploy the right scripts alongside the correct code. With that in place your team can start building automation around your deployments.
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Unfortunately the biggest "mistake" is architectural - your systems seem to require an outage to ensure code and migrations are in place before bringing the system back online. This may not be the fault of anyone - legacy systems are what they are and may never support in-place updates.
Ideally your migration code sits in the same repo as your code, so the same version will deploy the right scripts alongside the correct code. With that in place your team can start building automation around your deployments.