Dynamic subdomains for woocommerce affiliate sales but NO multisite. Is that possible?

It's as possible/impossible as you have time/budget for. It seems achievable to me, given that I'm assuming you're a for-profit organisation and this is generating you revenue somehow, but you might end up hacking some Wordpress internals at some point.

It's a broad and subjective question, which is not ideally suited to this stackexchange, but a couple of ideas below which are not 'solutions' just things worth considering.

1 - Use URL rewriting to make the subdomain a parameter passed to a single domain

Using e.g. the below mod_rewrite rules, taken from [here][1], you could perhaps solve some of your problem by getting the subdomain into a parameter you can easily work with in code.

The below htaccess (or something like it) would assume that you actually only have a single website at e.g. domain.com and a visit to affiliate123.example.com/foo/bar.php would actually appear to the server as example.com/foo/bar.php?sub=affiliate123.

If you make sure that all the URLs on your site are rendered relative to the domain (i.e. your URLs are '/foo/bar/baz.jpg' rather than 'https://www.example.com/foo/bar/baz.jpg') then the browser will consider them to be URLs in the current domain and you have a lot less stuff to worry about.

You'd then need to do some coding in order to be able to map this parameter in your URL that gives you the subdomain to lookup theme components, etc.

This probably has various issues that you'd have to figure out, like what happens if a plugin messes with your sub URL parameter or some other URL construction corner cases.

RewriteEngine on
RewriteCond %{QUERY_STRING} !^sub=
RewriteCond %{HTTP_HOST} ^(?!www.)(.+).domain.com$
RewriteRule ^(/?)$ $1?sub=%1

2 - What do you do when the affiliate wants to put some content on their page

Occurs to me that it's quite possible that an affiliate or someone in your organisation might have the requirement where they want some content specific to one of the affiliates to be posted only on that affiliate's page. I'd suggest you either allow for that now or make it clear that it's going to be impossible. If this is a possible future requirement, it makes Multisite a potentially much better option because of the ability to allow each affiliate to manage their own content.

3 Management tools

Seems like you definitely need to look into management tools that are going to let you manage a large number of similar sites from e.g. a centralised 'control panel' or set of scripts that you write. There are tools with plugins out there that let you manage many sites from a single dashboard. This could meet a bunch of your requirements. (This also mentioned by @TonyDjukic while I was drafting this answer)

Hope some of that helps you figure out which way to go. [1]: https://stackoverflow.com/questions/42263265/how-to-rewrite-subdomain-and-subsubdomain-into-a-parameter