I have a PPC l client who recently re-designed their e-commerce site. The only problem is that every single old link that they had before is now a 404 error. They had a fairly healthy organic ranking before as many of their site links showed up on the 1st page for a particular keyword - now that url shows up as a 404. They also have brand NEW links on top of that which are what I'm using for their ads.
I understand that these kind of things happen when old links aren't re-directed to the new site links and when the site isn't re-submitted for re-indexing.
My simple solution would be to do 301 redirects and then submit the site for indexing on GSC.
However while mining their site on Google, I've additionally discovered that their entire site is riddled with over 500+ urls of 404 errors - I'm assuming from years and years of doing the above.
I've read that doing bulk 301 redirects isn't a good idea, and bearing in mind that I'm not a dev specialist, what would be the best and simplest solution for this issue? Doing 410s instead?
Thank you for any help!