Help Tracking On-Page SEO Improvements with GA

Hi everyone, I’m a web developer with some knowledge in on-page SEO, but I’m new to Google Analytics and need some guidance. Recently, my cousin’s old brochure website was transformed into a Shopify ecommerce storefront by an agency - which, unfortunately, did zero on-page SEO. They also have a freelancer who handles Google Ads conversion optimisation stuff, and this dude is also not addressing any of the SEO (which I find odd, but I don't really know this side of the industry really well). Although the new site is functional and generating sales, it lacks crucial on-page SEO elements like alt tags, H1 tags, and proper meta descriptions. I’ve volunteered to resolve these issues, but I’m unsure how to effectively use GA to measure the impact of my changes. I’d like to track how my SEO improvements affect traffic - ideally, I want to be able to tell my cousin in a few months that traffic has increased from X to Y after my changes. Can anyone point me in the right direction within Google Analytics dashboard, explaining where to find this data that's relevant for me in this case? Additionally, are there any other metrics or potential pitfalls I should be aware of while monitoring these changes? Anything else you could advise to someone in my shoes? Thanks in advance for any tips or resources you can share!