Thanks again for sharing your ideas.
Yes, there are lot of alternatives.
Considering WPA2-PSK, it seems that long and random enaugh PSK is practically impossible to break. So, the simplest solution will be to have two SSIDs, connected to two VLANs - first for guests, opened at AP (no keys), and controlled by pfSense CP (vouchers), another one for employees with WPA2-PSK. Only problem is possibility that one employee gives key to others, but I think we can live with that.
Another approach will be to have all traffic going via CP. On that way, only one SSID/VLAN would be sufficient. I don't know exactly how CP is working, but probably it stores IP/MAC of user which successfully authenticated by vouchers or user/pwd. If this is correct, then it seems to me that it will be easier to sniff IP/MAC combination, and possibly misuse it, then to break WPA2-PSK. But I'm just guessing, I'm really not security expert. Also, if using plain http for CP where users enter their username/passwords, I think that credentials can be sniffed quite easy if using http. If, on the other side, I force https at CP, then I will probably have some issues about deploying root certificate, especially on some smartphones, etc. I know that same applies to vouchers for guests, but vouchers validity is measured in hours, so if attacker even succeed to grab the voucher code, he can use it same day only. Credentials for employees should be valid for much longer time.
So, these were just my ideas about various alternatives. At the moment, consdering all above, it seems to me that first alternative might be easier to configure and maintain, and "good enaugh" in my current scenario.