Here’s a tip, one of many I share for free on my other SEO website:

When applying keywords to a body of content, it’s not necessary to use every possible version or spelling, plurals, or ordering. Search engines like Google are smart enough to figure out that in New Zealand we will spell ‘realise’ with an ‘s’ instead of a ‘z’. Such variations do not need to be accounted for in your text content. Just write naturally and in the local version of spelling and grammar. Let the search engine take care of the variations.

You also don’t need to account for plurals or all verb paradigms either. For example: ‘t-shirt’ and ‘t-shirts’ will be recognised as a single word with two variants by Google. These will be shown in your Google Search Console account under the section about content keywords as a single entry. Similarly, any verbs are grouped based on the root, so ‘swim’, ‘swims’ and ‘swimming’ are grouped. Past tense (swam) is a little different for some (but not all) verbs, so include it where appropriate. The key is that you should use each morphological item as appropriate for the context. Do not feel as though you have to use each equally.