On separate PNRs (tickets) I'd be worried about only having 3.5 hours between arrival and departure.
If your inbound is late, and you miss your outbound you're out of luck - you may need to book a no-notice ticket. It's the risk one takes on booking separate PNRs.
I'd try and up the connecting time, even if it means a long, dull layover, because that reduces the risk.