Improve Gtmetrix & Google Speed Insight score

I need a professional to improve my Google Speed Insight score and GT Metrix.

I am curently using WordPress. I have alots of plugins ( 13-15) but i installed a plugin filter page to enable/disable plugin on certain pages.

I have a mutlisite Network.

I am good in SEO but my theme is very bad. There is almost no optomization, codding is a mess, i have also purchased a plugin called WP-Rocket but my my main theme has a /mobile path and when i try to separate the cache beetween pc and mobile some JS is broken. Also, when i combine JS, my mobile website theme is 30% broken..
. And also on pc version, there is a plugin sidebar not showing because a js file is broken... Also i know we have to defer parsing blok rendering i installed a plugin for that and also i bought imagify.io to minify image. I know what is. Webp and image optomization but i need help.


Please note that jquery.js is a big problem to defer because when i tried some menu was not openning in my mobile version.

Also, i need to auptomize font because i see in Google Speed insight that i need to fix some font size for user display...

I hope you will help me.

I dont know how much time it could take to improve my score to the maximum.

Thanks for helping me now.
Marie-Ève