Help passing Core Web Vitals - technical SEO

We're coming up to google's Core Web Vitals update, and although the site has some of the best content around, it's very much held back by site speed, LCP and CLS - especially on mobile. We're in the green for FID, but orange/red for everything else.

So for any technical SEOs that deal with backend issues, how much time / money does it usually come to for a client who needs to pass CWV and to implement:

1. Reducing CLS down to below 0.1 (and ideally 0, completely eliminating any shift). We're currently at 0.18 on mobile, but 0.3 field data on desktop, and 0.8 CLS for lab data.
2. Preloading LCP and taking steps to make other changes to improve speed (and ideally also improving FCP). Currently at around 3-3.2 seconds LCP.
3. Fixing elements that don't have one of a width or height element
4. Reducing DOM size (if possible - we host ads which affect our ability to use lazy loading but there may still be ways)
5. Helping prevent frequent Error 520 via some issue with Cloudflare
6. General site audit to find other areas of improvement.

It's not like the site has completely neglected speed or user experience - there are systems for caching, a CDN, etc in place. But, some technical help is needed to get the site over the line and pass Core Web Vitals, and for someone with the right knowledge I don't think this would take a huge amount of time - I just don't have the expertise.

So, any help / insight is much appreciated.