There are many ways you can incorporate nail art that does not involve special techniques. For example foils, swarovski, glitter etc. As for other types of more advanced nail art, or hand painted art, it requires practice. It also helps to have the correct brushes. With a striping brush, doing hand painted stripes becomes a walk in the park. You can't expect art of any kind - in this case nail art - to just happen naturally without practice (perhaps for a chosen few, it does). I don't mean any offence with this so please don't take it as such, but when you say you can't do it because you're "just not artistic", it kind of undermines the hard work, hours or practice and potentially money invested in technique classes and equipment that a lot of techs have had to endure to offer these kind of services. It takes more than just trying something out a few times.
Some people might learn faster. Personally I am a slow learner, as I have a learning difficulty diagnosis, but that diagnosis also comes with being incredibly stubborn, which worked in my favour when I wanted to learn some new techniques (like ombre or stamping, as you mentioned).
At the end of the day, if you want to evolve with the market, then it could be worthwhile looking into some classes. They can be anywhere from 50-200 pounds depending on the tecnique, length duration, who is hosting etc... If you feel that is expensive, try to perhaps dedicate a few weekends to practice, or an hour a day after work (or between clients). Start with something you feel your customers are asking for - for example you mentioned your stamping turns into a mess; what plates and stamping polishes do you use? I am sure many here can direct you to their preferred brand, and these things don't have to come with a hefty price tag. I was determined to be able to offer stamping, I think I practiced around 1 hour a day for a week (after work last customer left usually) to be able to stamp consistently. Having a look around for what stamper/plates/polishes people recommend is also a good idea. Stamping is really good as an easy way to spice up nail art, so if you wanted to start with something on the easier side, I would recommend practicing stamping actually.
PS: I recommend patterns, and mandalas, they are always very popular. I made the mistake of having just one mandala stamp plate in the beginning, so I recommend getting a few of those. Bigger ones that can cover the whole nail are so much fun, because no matter where you decide to stamp your mandala, it will turn out pretty - whereas other types of stamps will sometimes require some precision of where you place the stamp on the nail. I used to use really affordable Born Pretty plates and polishes when I started with stamping, it was cheap to buy and they worked great actually. I don't have a whole array of colours, just some basics, and black and white are most necessary. I have a few different clear stampers of different sizes, and some scrapers as well. I would recommend getting a couple different scrapers too, because even these can be down to personal preference, speed etc. Below is a link to a born pretty stamping plate I used A LOT. I'm not sure where in the world you live, but they have stores on ebay too.
https://www.bornprettystore.com/bor...a-55cm-manicure-nail-image-plate-p-40077.html