Filemaker Pro 18 crashes with Big Sur

I made the mistake upgrading to Big Sur 11.2.1

Now my Filemaker Pro Advanced 18.0.2.209 crashes when I select "Edit Layout" (using the Research Notes template). What is also interesting is that if I create a new document from the research notes template, it records the creation date as "April 29, 2019", not the current date of Feb 25, 2021 (I created a MS Word document to see if the same happened but MS Word recorded the correct date).

My problem is Claris/Apple do not support version Filemaker Pro 18 (I purchased this for $432 in June 2019, less than 2 years ago). I can upgrade to Filemaker 19, which they support, but it will cost me $264.

I understand it is possible to downgrade from Big Sur to the previous operating system, but I do not have the knowledge to do this and it does not look straight forward.

Does anyone have suggestion before I abandon Filemaker Pro and find a way to transfer my files to another product (with a better pricing/upgrade structure).