The Core

You can customize Wordpress to use a URL pattern which keeps the old extensions in place.

That's your best bet.

Unneeded change = unneeded risk.

The more things that change at once the greater the chance of an algorithmic rescoring that may not go in your favor.

This change is quite easy to do. Example shows .html, but you could do the same for .htm
https://premium.wpmudev.org/blog/how-to-add-html-to-the-end-of-a-wordpress-url/

If you do the above & it doesn't work...then depending on your server configuration & what Wordpress plugins you use, you may also need to have your server process .htm files as .php
   <FilesMatch "\.htm?$">
        SetHandler application/x-httpd-php
    </FilesMatch>
of course, always back up your .htaccess file before making any such changes to it.

I'll spare you the sad tale of a site relaunch gone bad in the last year or two (not a site I worked on, but I was asked to take a quick look a few months after the fact).

The key problem was that they had changed so many things that when they tried to figure out what was causing the huge traffic drop, it was essentially impossible to disentangle one change from the other. It was compounded by the fact that they had not archived ranking data (then Search Console went back only 90 days and they didn't have any other rank tracker).

Moral: the more things you change, the harder it will be to bounce back from a bad launch. Even if changing the URL doesn't *cause* a drop, it will make it that much harder to disentangle the real cause from the multiple possible causes.

Sometimes you don't have a choice because of CMS architecture (which was true in the case above with respect to URL structure), but even then, you might be able to phase things in.

Welcome to th3Core Revlis! Good to have you on board!

I'd would go pretty far in terms of keeping the old structure and if that includes .htm so be it. Google doesn't care about your URL's anyway and keeping a bunch of redirect rules up to date can be a hazzle. With Wordpress it's pretty easy to create the exact stucture you like - eg- keeping the old one.