Hello Christian
It seems that your NAT translation is not functioning. First of all, you did well to test all of the connectivity between devices as you did, and you verified that all devices are able to ping all other devices and subnets except for the PC. Note that with all your pinging, connectivity is established only via routing. No NAT was involved.
Now with your NAT configuration, you are not specifying which IP addresses should be translated. You have the ip nat inside source list 100... command, but you have not defined the access list 100. You mention at the end that you tried applying this, but it didn’t work.
Try applying the following access list:
access-list 100 permit 10.10.20.0 0.0.0.255
This will match all of the inside subnet for NAT translation. In any case, as you apply this, and as you attempt to ping, check the NAT translations using the show ip nat translations command to see that the packets are indeed being translated.
Note that you will still not be able to ping the PC from the PFSense firewall as NAT translations will only work for traffic that is initiated from inside, but the PC should be able to ping the PFSense, and the Internet as well.
I suggest you also follow step by step, the configuration procedure in the following lesson to more clearly understand each step that is taken:
I hope this has been helpful!
Laz