I think you chose well with Mint. It is based on Ubuntu, but has a track record of stripping out Canonical's nonsense, and if said nonsense should ever become impractical to remove, Mint already has a contingency plan in the form of their Debian Edition.
Someone said that LMDE is behind in various ways,
Someone on social media is always echoing the meme about Debian being unusable due to old packages, but roughly 96% of the time, that person turns out to be poorly informed and driven by an unhealthy addiction to quickly rising version numbers. Try not to give their opinion much weight, despite how loud and repetitive they are.
including NVIDIA graphics drivers
Why would "non-tech-savvy seniors" care what version of the Nvidia driver is installed?
Edit: To answer the question in your headline, putting the /home directory on a separate partition tends to make switching distros easy.
You already posted copypasta about this, two days ago, and it's still false.
Only Debian developers and Debian maintainers can create a Debusine repository. That’s not “PPA-like” in any practical way. The value of Personal Package Archives (PPAs) is that anyone can create them.