Link building and guest posting will help you to improve the DA of the site.
In starting you need to focus on increasing the number of referral domains and when you will have more than 1K referral domains then you can go for the other activities, For example, creating classified backlinks daily on high-quality sites.
To know more about it I have a great blog for you- https://www.ezrankings.com/blog/how-to-increase-your-websites-domain-authority/
Hope it will help you!
If you still have any queries then you can call - 9560133711

Here are few steps for SEO audit-
Identify link-building opportunities on high-quality sites.
Optimize meta tags.
Find potential information architecture improvements.
Find and fix thin content.
Run page speed analytics