Should I switch to a responsive format if my site is mobile-friendly with a separate format?
It’s a case by case basis, but it’s always better to make changes if possible.
But one of the biggest problems, especially for a huge site, is time.
From March, when the announcement was made, there will be a mandatory transition in six months at the earliest, but that may be too short a time to plan and implement any changes to the mobile-ready format going forward.
Unreasonable planning itself may have a negative impact on SEO, so you need to do a careful study.
Depending on the site and operating conditions, it may be best to leave the current separation and adopt measures to thoroughly close the differences in content and settings on the desktop/mobile pages by September.
You may refer to this guide for more info around what google likes and how data is served across web and mobile platforms
https://developers.google.com/search/mobile-sites/mobile-seo?hl=en