Starting my own SEO agency: Key parts of an SEO-plan?

At the moment I am still in the middle of my study Commercial Economics. My ambition is to start my own marketing agency (SEO-specialized) after completing this study.

Now my path has crossed with someone who has his own marketing agency specialized in SEA. His clients need SEO but he doesn't have the knowledge to provide it. He sent me a message asking if I like this. This is of course a perfect opportunity to start my business. My intention was to start quietly with just writing SEO content, but he requires a complete SEO optimization plan for his clients. Because I'm only a student and SEO is mainly something you learn through experience, he wants to test my skills and I got a (paid) 'trial assignment' for one of his own websites. So I'm thrown in the deep end and I would like tips on parts that should NOT be missed in this plan.

For this plan, I have acces to his Google Search Console. The rest I have to figure out using F12 inspect element.

Currently, the contents of this plan are as follows:

• Keyword research
• The use of H1, H2 (etc.) tags
• Optimization of meta descriptions
• alt text for images
• URL link structure
• Internal link building
• Pagespeed, however I don't really have experience optimizing this

Do you have any additions or tips at all for this assignment? I like to hear it!

TL;DR: CE student with aspirations to start my own SEO-specialized agency. Someone who knew this (owner of a SEA agency) reached out to me and asked if I want to do SEO for their clients. He requires a complete SEO optimization plan, but to be honest I mainly have experience with writing content and the above components. I have received a paid trial assignment. I would appreciate it if you have any tips about this assignment and the contents of this plan.