-- Is this approach appropriate?
Yes. Leave everything in place, just disable the other extensions until you're happy with the results. Once the site is live and working, then you can think of uninstalling.
-- What do you recommend for this type of effort where there are already tools in place that duplicate some of the functions within 4SEO?
Sitemap: 4SEO needs to analyze the site in full so that it can produce the sitemap, any update later on is automated. So you can have the first analysis happens on your staging site. The data collected is relative to the root of the site (ie the domain is not part of the data) meaning when you move it to the live site, you'll only have to change the site address but the collected data will still be valid.
Structured data (tassos): depends on your current setup. If talking about Joomla articles, 4SEO has built-in rules so there should be nothing to do (aside from disabling GSD), unless you use specific features in GSD to build custom-made structured data records. If other extensions are involved, a review is needed to see if 4SEO supports them, what GSD rules are in place.
.htacces redirect: You can create redirect rules in 4SEO to match those in your .htaccess. But in that case, I'd argue if your .htaccess rules are there and working, I see no point in re-creating them inside of Joomla. .htaccess will always be faster to execute the redirects, while 4SEO will always be easier, to create and to manage. So it depends a lot on who you want/can manage redirects, whether you have a dozen or a 400 (you probably should not have 400!).
One thing I'd avise is to make a decision and use one or the other. Likely not best to split between 2 tools to manage redirects.
-- Can the metadata in JSitemap Pro be imported to 4SEO?
Sorry no. We're currently extremely busy completing 4SEF, our successor to sh404SEF for SEF URLs management, and extending 4SEO to add the ability to import meta data and redirects from sh404SEF.
JSitemap was never really an SEO extension, most people use it for sitemaps only so we have not had any prior request for that actually but importing meta from JSitemap will happen, I agree it's important to facilitate the transition. I have added this to our internal tracker but unfortunately cannot provide any timeline.
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