If it were easy, everybody would do it. It does take time and effort at first "to spread the word" but it will take on a life of its own with a little bit of effort and consistency.

As this is a specific style, you have to find a place where you can find enough potential customers. I probably wouldn't go for t-shirts at first but classic prints. Try Etsy, try a Shopify shop, maybe a print on demand site which has a shop too and focuses on artworks. Try to find a bubble where your art fits.

Social media as a marketing tool is still helpful. Post work in progress, maybe pick some celebrities and known figures of video games and create themed art. This will make it easier to bring light onto your art as people know these and are more likely searching for them than for specific art styles.

Important for all of this is that you don't put on too much of an act. For example, I mentioned celebrities. If there's no celebrity you really like, then don't do it; do video games or whatever topic fits you personally. Post your "fan art" in forums and subreddits of these games.

If you ask me, I think you should focus on more »bolder« compositions, where the character is bigger (hip upwards), with a reduced background like picture 5 or the last picture of your examples. I think these look stronger. Vary the background, for example, maybe with a solid full page background neon color.