Help to get back my rankings


I have a website on wordpress that had in 2018 and 2019 very good rankings for all searches related to holidays.

A damn security plugin put all my pages in soft 404 at the end of 2019 and it took me 3 months to find out where the 404s were coming from. Now I deleted the widget and all my pages are indexed again but I can’t get my old rankings back.

I published a lot of content and it seems to me that my site respects SEO standards. I am thinking about changing the theme I use to make it more mobile friendly. But I don’t know if it’s worth it.
Any advice ?