I'm still running Mojave for both RM7 and RM8. I could upgrade to Catalina, but my iMac is late 2012 so it won't upgrade to Big Sur. The only reason I am staying on Mojave right now is to support testing in the older versions. We have employees on different MacOS so we can test on them. Our programming tools for RM8 say the earliest target Mac is Sierra, so anything older we won't officially support. However, I have seen users on El Capitan. For RM7 users we have the older wrappers that can be downloaded if the Big Sur wrapper isn't working on their older Macs.
When you create the GEDCOM under "People to export" use "Select from a list". Highlight your mother and then click on Mark Group. Select "Ancestors of highlighted person" and on the screen that comes up select "Ancestors and descendants of ancestors". Set the number of generations for both Descendants and Ancestors. What this will do is climb up the tree that many generations of ancestors, then it climbs down the tree of each ancestor that many generations. It will get the full tree for the person. The only people it won't include is anything that goes up the descendants spouse's line. It will stop at them.