I'm glad that you've found a workaround for your problem.
Thank you for letting me know.

Here are some further notes on the issue...

As you have discovered, WP-SimpleViewer is incompatible with WordPress 5.0 and later (due to the TinyMCE editor being replaced by Gutenberg) unless you revert to the old editor using the Classic Editor plugin (written and maintained by the WordPress team).

Unfortunately, due to the demise of Flash, SimpleViewer has now been retired (since July 2017) and there will be no more updates.
Please see our blog entry entitled The End of Flash and What this Means for SimpleViewer for further details.

With this in mind, I would recommend going forward with Juicebox, our HTML 5 image viewer (which does not rely on or use Adobe Flash at all).
WP-Juicebox (the Juicebox plugin for WordPress) has not yet been updated to be compatible with Gutenberg yet (at the moment, you would still need to install and use the Classic Editor plugin) but it will be updated for WordPress 5.0 and later (and, being HTML 5 instead of Flash, Juicebox does not suffer from any of the Flash-related problems that affect SimpleViewer).

Unfortunately, SimpleViewer has been retired for over a year and a half now and I think it is very unlikely that any further coding relating to SimpleViewer will be undertaken.
Also, creating a plugin to convert WP-SimpleViewer galleries to WP-Juicebox would be a huge task. There would be a lot to consider: different configuration options, different database tables, different embedding code, different gallery data files.

However, it might not be as difficult or time-consuming to convert a WP-SimpleViewer gallery to a WP-Juicebox gallery as you think.
If you install WP-Juicebox and use the plugin to add a Media Library-sourced Juicebox gallery shortcode to your page or post (just as you would with WP-SimpleViewer), then Juicebox will display the same images with the same captions as SimpleViewer would (i.e. those images attached to the page or post). There would be no need for any further image or caption management. (You could then delete the SimpleViewer gallery from the page or post.)
Also, you could create a set of default configuration options for all your Juicebox galleries on the 'Manage Galleries' page and then, each time you create a new Juicebox gallery, these options would be used (so you would not need to manually configure each and every gallery).

I hope you get on OK with WP-Juicebox if you decide to use it.
If you run into any difficulties, just give me a shout over in the Juicebox forum and I'll do my best to help you out.

Incidentally, WP-Juicebox v1.5.1.1 has just been released with compatibility for WordPress 5.0 and Gutenberg.