I got a spammy testing website back by using a similar strategy,
- removing all the posts & placing a 410 header
- changing and improving the website theme, design, etc
- adding a well detailed about us and author pages for better EAT
- publishing new versions of the old content, longer and way better quality
- i've used the exact same titles with different urls tho
OP, my strategy might or might not work for you,
I've done that with a testing project, no serious intent to grow any further, just made to "test the water",
You should focus in getting better on-page overall, some decent links might do the trick as well,
Nobody can specifically tell you what to do without having more information about the website