Ahh yes, the well known dilemma of "data portability", also know as "If you can't leave with your stuff, you might as well stay with us". That's something I'd definitively recommend you look out for in the future, here, I'll make the first step for you [for Joplin]:
Joplin stores all your notes on your device and allows you to export them in several nonproprietary formats, including markdown and HTML, which are human readable and directly importable by generally all open-source note apps. Joplin being open-source helps too, as it means that anyone can directly add new ways of exporting notes into different formats should you ever want to switch. Joplin is not perfect since it still changes your files during usage, but one could argue that it's well within reason since it adds several features on top that the raw markdown format doesn't have.
As for your problem at hand, imma be honest chief, it's not going to be perfect. You have two options, but both of those options will require that you manually adjust some notes, that's just a consequence of today's world in which different note apps are built completely different and there not being a universally agreed on format that can easily contain all the contents of a single note in one file. Synology using their weird format doesn't make it easier either so you're going to have to put in the work to break out of that file format first. This is true regardless if you ultimately decide to switch to Obsidian, SilverBullet, MoeMemos, Nextcloud Notes, you name it. With Joplin at least, you'll be able to automate the import of 98,9% of all your notes, but even that still means that you'll have to manually adjust some notes. Here are your options:
Automate the process:
There are several scripts that automatically take the Sinology .NSX export you just created, then they translate all it's content into .md files that you can then easily import in one-click into Joplin. Here are the ones I've found:
A customized version of the above script "to export notes/images in markdown + Front Matter (Directory) that can be [more] easily imported into Joplin."
This script might be worth it if the above scripts don't work. Some people had issues with this script, but managed to mostly fix them. See here.
Copy-paste each note:
This sounds tedious at first, but once you get in the flow, it isn't that bad. It isn't doable if you have 10'000+ notes, but in my case, I got it in a few hours. Remember that even if it takes you one hour a day for a week to move them all, since you're switching to a nonproprietary format you only have to do this once and then you're set for life. This person on the Synology forum had your same problem and ended up choosing this option.
Lastly, my personal experience: I moved from Google Keep to Joplin and I know nothing about scripts or code, so I copy-pasted most of my notes manually into Joplin, downloaded the attachments and added them manually, then reformatted the notes manually. It was a pain in the ass. But nowhere near as painful as importing 1000, 20'000, hell possibly 100'000 notes that will probably accumulate in the years to come. Importing them in a different note app would be straight up inhuman or at the very least impossible without a script, so I'd personally recommend you and your family make the switch to a more flexible file format right now, while you still can.