There are definitely some downsides of taking a shortfall command early on (even if it is available to you). I'm talking about those who achieve their command within the first 2/3 years in BA, which is historically unlikely anyway.
1. Zero or close to zero roster control (indefinitely)
2. Frequent reserve periods
3. One of the lowest command salaries in the UK for an A320 command.
4. Potentially years of stagnation as senior pilots transfer across ahead of you
If an A320 command is your thing and you're not interested in long haul it makes zero sense to leave somewhere like easyJet IMO.
If you join as an FO anytime soon you have a decent chance of achieving some lifestyle control pretty quickly based on the latest estimates for recruitment. What you then do is up to you.
As was said to me when I joined, you only HAVE to be junior once.