How long were you on how much allopurinol per day before that 6.0 result?
And are these three measurements are the only ones you’ve got?
I assume you have since then stopped taking colchicine. Side effects can become serious over time.
There are no official warnings for not taking drugs but obviously it doesn’t mean failing to treat a disease is necessarily safe.
In particular, I’m concerned about these kidney stones. I’m not knowledgeable about kidney problems but I know some people take allopurinol or febuxostat for kidney stones because uric acid is one of the things which can form stones. Taking too much of these drugs can in principle also result in kidney stones (composed of xanthine). Young people would normally not get kidney stones from uric acid when it tests under 8 but even though your values aren’t very high, possibly the condition of your kidneys might make values as low as yours dangerous. So a diet designed to lower uric acid and help with its excretion could contribute to protecting your kidneys. You would probably also benefit from taking some drug but the simple drug-only approach may not best in your situation. You may also benefit from taking smaller drug doses than most people.
Have you ever seen a doctor knowledgeable about kidney issues and did you discuss uric acid with them? Or have you read the medical reports about your kidneys and the composition of your kidney stones and researched these issues yourself? Obviously there’s a lot more to kidney function than whether or not you have a history of kidney stones…
There are a few other drugs than the ones you’re listed as well as other compounds sold as medicine in some countries or simply found in foods which could help with your uric acid problem but you’d have to be careful about their effect on your kidneys in particular.
Sometimes serious drugs are best in spite of their side effects if one uses them carefully because one may only need a very small amount in comparison to less specific or less effective medicines.
About febuxostat and your history of tachycardia, there’d be another balance to strike regarding the risks but I think your kidneys ought to be considered first and I lack the knowledge or experience to help you with that. If it turns out your kidneys would benefit from a drug and/or diet which happens to alleviate your uric acid problem as a side effect, there might be no need for you to try febuxostat or allopurinol desensitization.