Your site appears to be pretty well structured in most ways and you're getting a good score. If you're seeing a huge swing day to day, test to test, etc, then it's probably a hosting related issue. What type of hosting are you on? Is it cloud, is it shared, is it WordPress specific?
Google's PageSpeed Insights rates you at 93 for desktop - that's good. In fact, the only place it even throws a complaint is Reduce Initial Server Response Time: 1.73s. So what that tells me is that the inconsistent performance scores you are sometimes seeing are probably down to your hosting.
There's a few key things to remember when you're evaluating a site's performance, because there's really three different 'performance areas' that you need to consider.
There's your 'user perceived' speed; when you open the site in a new browser on a new computer or a new device and visit the URL directly for the first time, what's it load like for the end user in terms of what they see and what their experience is. This experience is super important.
Your page speed 'rating' - this isn't necessarily a direct indication of how fast the page loads, but rather a diagnostic check of how everything involved in generating a page is optimized and yes, Search Engines do consider these values.
Theres your 'actual page load' speed; as in how quickly the entire contents, code and assets of the page load.
If your Page Speed Rating is good and your user is seeing a full screen of useful content almost instantly, then it's ok if your site is still loading future needed assets and content. Obviously we want this to be as quick as possible but if what's loading last is a script that validates a MailChimp Subscribe form that pops up when a user clicks a button and only after the user has typed out their info, then it's not a huge worry if that script loads a half second later than the page is visible to the user.
You have to find a balance and ultimately test your site as a new user and measure the experience there first, then look at your Page Speed Rating and lastly look at your actual full load time, that's the order I address things in.
My recommendation, if you want consistency in performance and you want a really smooth running site is to avoid those plugins that promise better performance, automated minification, etc... ...I have seen the mess that can occur when those things go wrong and it isn't pretty. And those plugins also aren't a 'fix' for a slow or broken site. Those plugins, IF used, should be used to take a well built site that functions very well and then simply optimize what's already working. If your theme or a plugin are running really slow, laborious queries over and over again, there's no amount of minification or caching that's going to fix that.
So drop those plugins and get yourself onto a really good dedicated WordPress hosting platform, especially if you're running eCommerce. I like to use FlyWheel and then hook my site up to ManageWP for hourly back-ups when it comes to eCommerce sites so I always have all the sales data backed up. (Not linking to either one because that may be against the rules here.)