Plugin will be the recommended option.

Plugin is easier to install and just go for it. Running a sub directory will require more wordpress and name server setup knowledge including potential secondary wp installation.

I host 1000s of WordPress sites.

Here are some rules to keep your sanity working with WordPress.

1) Never. Ever. Ever. Use any type of specialized theme.

2) So, avoid using a Question + Answer Theme.

3) You'll avoid specialized themes because...

a) This type of theme's coding is usually substandard, unless you find it in the WordPress theme repository.

b) This type of theme becomes abandonware (developer's die or lose interest) at some point.

c) Changes in PHP + WordPress internal APIs usually kills this type of theme in some way, so at some point a PHP or WordPress update will kill your site.

4) Do use a solid + simple theme, like GeneratePress which lives in the WordPress repository, so must pass all the PHP + WordPress unit tests, before any update becomes available through the repository.

5) Do use a Q&A plugin + every plugin must be vetted.

For example, when I client asks me to vet a plugin, I do these steps...

a) Run phpcs on the plugin to ensure it works with PHP-7.4 (latest stable) PHP.

b) Install the plugin on a test site + go through entire setup, then check WordPress debug logs for diagnostic messages.

c) If #5b surfaces minor fixes, then these fixes are requested of the developer + if the developer fixes their code quickly + correctly, plugin testing continues. If developers are unresponsive or unable to fix their code, this plugin + company are blacklisted from any future use on any site I host.

d) If plugin produces horrible problems in the WordPress debug log, then the plugin + company are blacklisted from any future use on any site I host.

6) If the plugin is in the WordPress repository, check the issue tracker for the plugin + related company. If the company fixes all support issues quickly, you likely have a great company to work with. If you see issues which are months or years old with no response, this plugin + company best be added to your blacklist.

Summary: Use WordPress issue tracker (when possible), then phpcs + the WordPress debug log to determine which plugins may be useful.

Short answer will be: you don't need any specific tools for this. Just use Gutenberg (in-built editor) to style your Q&A page and that's it.