I did a little poking around the internet this morning to see if I could find an alternative program. While there are some alternatives out there, users in your position seem to agree that the alternatives are missing some features. See this thread, for example.
Greeting Card that will work with Sequoia… - Apple Community
A couple of potential solutions occur to me. They may or may not be worth the effort depending upon how much you use the program. I can't test these to make sure they would work because I don't have that program. These are ideas that might work.
1. Create an external drive with macOS Sonoma on it. That version will run the Hallmark software. This would require you to boot the Mac from the external drive any time you want to run the Hallmark software.
2. Install Sonoma, or other OS that can run the Hallmark software in a virtual machine. Then install the Hallmark software in the virtual machine and run it from there. Virtual machines do not require rebooting the Mac but aren't great for some computer-intensive tasks.
EDIT: Minor edit to correct typo and clarify one thing in option 2.
