I was executor for my mom’s estate, too, and I took a very similar approach to the one described above by @james.cutler, with one twist:
I created a separate user account on my Mac to handle all estate-related matters, and I created a separate email account for the estate, as well. This made it very easy to avoid accidentally mingling estate information with my own personal information, and it was much easier to search for information in this self-contained environment than if my own personal data was in there, too.
I did use Quicken in that separate user environment to download and consolidate most of my mom’s financial information.
The strength of this approach is that I simply used the same tools that I used to manage my own financial/legal matters. No need to learn any new software.
Separately from tech issues, if you haven’t consulted an attorney who specializes in estate matters in your mother’s home jurisdiction, I do suggest having at least an initial consultation, which often is free or at a nominal fee. Every jurisdiction is a little different, so an experienced attorney can help you to identify key actions and requirements so that you can create a checklist for handling the estate without necessarily spending a lot of money. Also, many jurisdictions will be able to provide an outline of required steps, often detailed on their court websites at no charge.
In my mom’s case, the biggest surprise was discovering that she had opened quite a few bank accounts over the years, some of which ended up in “escheat,” i.e., the state’s unclaimed property fund. While the amounts weren’t huge, recovering those funds did make a difference for the estate.
You have my sympathies. Best wishes.
Edited to add: PS. For the email account, I used a paid Microsoft account and stored scans and other documents on Microsoft OneDrive.