Lighthouse does not have a single alternative because it is several tools in one panel. Replace the panel you actually read.
Performance alternatives
PageSpeed Insights adds field data. WebPageTest is the alternative when you want a filmstrip and a repeatable lab setup. Keep one of those for speed. Do not replace them with an SEO suite and hope the score comes along.
SEO alternatives
The SEO category inside Lighthouse is a handful of on-page checks on one URL. Fix SEO Today is the alternative when you need the whole site: indexation, duplicates, internal links, and GEO. It is not a lab trace.
Accessibility alternatives
axe and a dedicated accessibility audit are the alternatives for WCAG work. We are not that audit. We will not label a crawl as an accessibility API.
Not for performance engineering. Yes for the question Lighthouse SEO cannot answer: what is wrong across the site.
Should the Lighthouse SEO score be 100?
It is a small checklist. A perfect score can still be a page that should not be indexed. Crawl the site.
Why is Fix SEO Today cheaper?
Lighthouse is free. Replacing the extremely expensive suite beside it is the actual decision. Fix SEO Today is cheaper because you pay for the diagnosis. A free peek, then $19 a month for one project or $99 a month for unlimited projects. That covers the crawl and the plain-English fix list, including GEO. It leaves out the backlink index, the rank-tracker seats, and the ads module.