You can also ensure that not many things change over time. My parents have been using Linux for over 15 years and they love it. Mate looks basically the same, as do most of the applications they use (file manager, image viewer, Firefox, Thunderbird, etc.). There's no strange software required for their Brother printer/scanner and nothing suddenly appears with each update.
This how it works or most people in the world. They get a phone and they change it only when they are forced to (hardware failure, some app they absolutely need stops working etc.). The group of people changing phones because they are no longer supported with security fixes is very small.
This is beyond stupid how fucked up the phone market is. I'm still using my 2008 laptop with newest LTS (x)Ubuntu, but my 2018 Android phone lost official support in 2020... Thanks to that, now I have iPhone (longer support) and Fairphone 5 (I like how they try to upstream everything for this phone and promise really long support).
About your question:
Should be fine. As long as browser is up-to-date, she doesn't connect to unknown WiFi networks/Bluetooth devices and she is not targeted. The easiest security fix is to not have anything worth anything on the phone :D
I'm not claiming that there is no use for Jack. I just think that this is a niche and doesn't require a separate connector in every smartphone. Having a universal connector that can be used for various niche scenarios is IMO enough. We should push for improvements in that regard (e.g. make it easy to charge the phone and use the dongle at the same time).
I wish I had the same problem as you do with your car. Mine is old enough that it doesn't have Bluetooth (I'm not the first owner and Bluetooth was an "extra" in this model) but new enough that there is no cassette player (and no built-in aux input) :)
I personally know absolutely no one with a dongle. I know many people who still use phones with jack that are not using it (or have earbuds). I was using wireless headphones before anyone heard of "earbuds" - they are much more convenient in situations where I'm using headphones. Of course, it's anecdotal but it seems that there are much more people like me/my friends than people missing Jack...