first, I struggled with the OPC node limit of my Siemens plc. but now I have the problem that the new tags that I have added aren't readable because there are too many monitored items in the subscription. Does this have to do with that I'm using the trial of ignition and that I have to buy the license if I want to test further or something else?
No, this is a limitation of the server you are connected to.
Try creating a new Tag Group in Ignition and add more tags there. Each tag group is an OPC UA subscription.
Depending on the model, there is a property you can edit to increase the number of monitored items/tags you can poll. This page covers the limits, unfortunately, I don't remember the exact path in the settings to where you change the default limit, but I believe it is on the same page that you enable the server on.
Edit: Found a screenshot