I've been at several startups that used PEOs and founded two.
A PEO has nothing to do with your culture, and almost nothing to do with your onboarding. It handles payroll and benefits for you. Some try to provide communication or project management tools, but every company I've been at has completely ignored those.
So on day 1, the employee brings in proof of identity and work eligibility, fills out a couple of government mandated forms, provides direct deposit information, enrolled in your benefits plans, etc. That would have to happen regardless of whether or not you use a PEO. The only difference is that they have to create a login on the PEO's site.
That's the tiniest portion of onboarding, if you're doing onboarding right.
How do you think a PEO would interfere with developing your own culture?