They are totally different axis.
WP is mostly small business workhorse, nearly all included in package (admin, editor, lot of third party addons), which is good enough for small scale, cheap and simple (most things could be done without developers), but not scales well.
So if business grow, at some moment it will be very hard to use WP, and/or business will be slow down if continue use WP.
Django is better when have developers team, it is full featured framework, have many things included into one officially supported package (but not as complete as WP), optimal for medium business, and with good developer/ops, could serve any scale, but it have too steep learning curve for start small.
Flask, could be considered as simplified Django, also good, but to make same things as Django, you need to create modular design from many separate packages, which are not orchestrated/supported by one official source.
So with Flask you probably will start faster than with Django, (I myself spent ~half of year to learn Django, with Flask I started in weekend), but you may encounter problems, when some dependency will be changed too much, so you will need to recreate interface for it.
Unfortunately, Django does not offer standard way to transit between major releases, but within one major release you could expect minimal efforts to transit, if use standard components.
Many things, possible to recreate on other version, just by change config and/or database scheme, so many works could be done by ops (Admin/Database administrator).
WP also does not offer standard transition way, but in many cases it is just very simple to recreate all from scratch, many works could be done by just user and some things need ops.
With Flask, when you need to transit to other version, you probably, will recreate all interfaces between components, need programmer to do this, also ops.