It always depends on many factors. You cannot successfully compare one keyword with another that has a good position in the search results. Where one word has only little competition, this can completely differ for another word, so that the approach for ranking will be different.

Quick solutions are often fairytales. Seo is a long road without end.

Your "supplements to boost immune system" is in 3 alt tags only. Google can consider this as keyword stuffing. Try to add it also in your decscription. and in one H2 tag. If this is the main keyword for the page, add it in your meta tags too. Then link from another page with the exact text to your page. And if possible with some varitions from other pages.

A few backlinks from relevant high authority domains will do the rest.

By reading a little I thought everything is great you just need to research keywords and rewrite content.

Best ways to find.. Try to find your competitor then coolect those sites where they get link from.

You may try below steps. Hope you will get some quality link.

1. Find relative sites who writes in your niche
2. Collect there emails
3. Send them proposals