Hello @saimrasheed
Thank you for your feedback.
The ttfb may vary and every time I test the performance the result is awesome.
GtMetrix has a partnership (paid) with the mentioned plugin and naturally, they are suggesting using that plugin for your website.
Your website score is awesome and as for your other questions:
Avoid an excessive DOM size – Not related to W3TC or any Caching plugin. Preventing an excessive DOM size for your webpage isn’t easy as there is no one-size-fits-all solution.
When you’re building your website, it is a good practice to integrate certain processes in your development workflow so that your production website doesn’t suffer from an excessively large DOM size.
For example – when using a CMS to build your website, pay attention to the themes or plugins used as they may inject too many elements, some of which may be unrelated to your site functionality.
In many cases, these features are hidden, making you think that there isn’t any code. Where possible, remove unwanted code that is irrelevant to site functionality.
Reduce unused CSS/JS – No plugin can do this successfully. The reason for this is that many themes and plugins are loading the JS and CSS files on all pages and not just on pages intended for. For example, if you are using some plugin for your contact form, they may load CSS file on all pages instead of just loading it on the /contact/ page
Serve static assets with an efficient cache policy – W3 Total Cache does perfect job here. However, you do have some files that are loading on your website from a server other than yours (3rd party resources). Since those files are not on your server, W3 Total Cache or any other plugin cannot add expires header or cache-control policy to those files.
Once again, the score is almost perfect:

Thanks!