This is a slightly complicated question, but let me break down my response. (full disclosure, I work for Pinegrow)
So, for a good period of time I built sites using WordPress and the Divi theme. This is fairly comparable to Oxygen. With regards to putting together pages. Pinegrow does require a bit more HTML/CSS knowledge as compared with Oxygen. If you use a framework like Bootstrap there are a number of good blocks to drag to the page much like Oxygen.
So, if you feel somewhat comfortable with the HTML/CSS portion there is the WordPress portion to consider. Oxygen is very much a WP page builder. Pinegrow is a full WP theme builder. They are really very different beasts. With Pinegrow you can add custom post types, add options to the theme customizer. However, along with this does come a requirement to partially understand the WP codex. Much, much easier to use Pinegrow to make a theme than to make it from scratch, but some knowledge required. For some in depth theme options you need to have a little PHP knowledge. So yes, Pinegrow is a very good “gateway” editor to making themes until you better understand the WordPress universe. Then it becomes an even more powerful and quicker way to make themes!
Once you get used to the workflow, Pinegrow can be used somewhat like Oxygen. Working within a given theme you can generate specific pages.
Pinegrow produces standard WordPress pages, so any plugins that aren’t theme specific will work with themes created by Pinegrow. If the theme you are putting together uses a plugin that uses shortcodes, no problem. Once your PHP is a little better you can also write your own shortcodes like any other plugin/theme.
If you find that your clients do not need the full CMS of WordPress then moving away from it would be fine. I should point out that JS is a great add-on within WordPress, as well. With regards to Pinegrow, it plays well with JS on straight HTML pages. It doesn’t play well with JS-based pages, like react, at the moment. Pinegrow also has some CMS features for clients that need more of a WordPress lite site.
Finally, we are pretty helpful in the forum getting people up to speed. I won’t lie, Pinegrow can be a little daunting at first, but really helps with learning straight HTML/CSS page and WordPress theme creation.