Site Redesign and SEO

Rookie AF question here... roast at will I can take it...

I have a client with a 7 year old business and lots of long form content, but not super SEO driven design or mobile friendly. Wants me to update site, give more modern feel and mobile friendly.

They have done a good job of the GMB setup and have a 4.9 on reviews on google with over 200 legit client posts on it.

I am wondering just how badly I will kill their rankings if I nuke the site and redesign it from ground up? It's been a while since I have done this (I moved into ERP and Systems consulting a while back) and I want to make sure that the new site doesn't drop their search rankings.

Any great tools I can use to decipher what they are actually ranking for and make sure I hit those hard?

I've been reading up on the GA4 stuff and it seems like so many items are added into ranking now.

Help me win on this one.. I want to use it as a jump off for maybe getting back into website development.

TIA

Create a mockup of one of the page get it signed off and make it live. Do the same for rest.
Don't change content don't change image don't change url don't slow down the page don't make the site unsecure. Rankings will be fine.

simple bro
Make a whole website with same links and contents and tags etc on dummy domain and once it all done them quickly down the website and upload this version of that website immediately

in this way the ranking wont go much and redesign done,

Copy all the content from your old domain to the new domain, prepare the proper redirects, and ensure that all of the existing content links are working.

You can use a plugin like Duplicator to duplicate your existing WP website and replace it with your new theme.

https://nl.wordpress.org/plugins/duplicator/

I can do that for you, just let me know is you still need it.