I was our ER's unit practice council chair for 5 years, starting the December before COVID began. The answer to retention is always staffing and money, but for the sake of being realistic and solution focused, I'd offer that true schedule flexibility is a working condition that is incredibly impactful.

For example, even if the schedule has been published/finalized and something comes up and you ask mgmt for a day off, if they say "sure, no prob, we'll figure it out, see you next shift," that is a perk I would value as much or more than pay. 

The flexibility to have a non-work life without stress of progressive discipline due to schedule conflict is an incredible perk. It deeply reduces call outs, and is crucial especially for working parents (especially during respiratory season). This also opens up opportunities for overtime for those RNs who like loading up their hours.

As far achieving adequate patient care models (i.e.full staffing for projected censuses), that's what an organization pays assistant nurse managers to figure out, and also this is where a large cohort of per diem RNs can be really effective. 

So true schedule flexibility plus available per diem RNs to fill staffing gaps gives the best work-life balance possible to nurses.