The 30 device limit is per OAuth. You can add additional OAuth apps if you have more devices. I have four OAuths authorized myself, but they were created before the limit was raised from 20 to 30 devices per OAuth.
You can mirror Hubitat→SmartThings by authorizing Hubitat devices in advanced configuration, which can be enabled immediately under the OAuth That will take you to this screen, where you can "mirror" those devices to any available SmartThings virtual devices.
If you need assistance, I suggest posting in the HubiThings Replica thread. The developer and others are there to lend their assistance.
It's worth noting that HubiThings was developed with an emphasis on mirroring SmartThings devices to Hubitat. If you're going to do a lot of complex mirroring in the opposite direction, you might want to consider using Mira, which was developed for that purpose. Note that you can easily use both if that suits your needs.
Hope this helps.
