With admin access alone, you can handle almost everything SEO-related:
Add/edit meta titles & descriptions (via plugins like Yoast/RankMath)
Optimize on-page content (keywords, headings, internal linking)
Manage XML sitemaps
Set up 301 redirects
Implement schema markup (via plugins)
Optimize images (alt tags, compression plugins)
Control robots.txt (many plugins allow this)
Connect tools like Google Search Console & Analytics
Only in more technical or edge cases:
Editing .htaccess manually for advanced redirects or rules
Fixing technical errors that break the site (plugin/theme conflicts)
Adding custom schema or scripts directly to theme files
Resolving site speed issues involving server-level changes
Recovering access if the site crashes after an update