the more challenging (but maybe easier, too??) of writing my own "Postgres" module that actually acts as a *wrapper* to the current DBI module
I don't think this is a good idea. To access a Postgres database using DBI, you'd need the DBD::Pg module. This converts standardised DBI calls to the Postgres API. I assume that the existing Postgres module looks a lot like the Postegres API (that's how most of these older database access modules work). Your suggested module would therefore convert Postgres API calls to DBI calls, only to have DBD::Pg turn them straight back again. That pretty much seems to defeat the object of using DBI to me.
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