There are no proof of any new variant as there is no virus isolated. Even the ‘fact-checkers’ cannot say that, because it isn’t isolated.

This is completely false. It takes less than 2 minutes searching on the internet to determine this is false. Scientists have genetically sequenced all the well known variants of covid 19. The original one was sequenced at the beginning of the pandemic. Here's an article discussing the sequencing of one in Korea, but it is easy to find more of them: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7036342/

And since you invoked fact checkers, here's a fact-check article that discusses the root of the misinformation on this topic: https://www.reuters.com/article/factcheck-covid-rna/fact-check-sars-cov-2-has-been-isolated-and-its-complete-genome-has-been-sequenced-idUSL1N2LS27P

Even if there were new variants, all the medical science until 2019. knows that mutating viruses are less and less dangerous with every new mutation

This is also not true. Random mutations can be more or less dangerous than the original. That is just common sense, the changes, after all, are random.

As to the variants that persist over time (due to "Darwinism"), they can sometimes be less deadly (because the most deadly can wipe out their host species entirely in area, making it more difficult for them to spread). But they can also be more deadly, if they originally weren't super deadly to the point where they rapidly kill hosts before they can spread the virus. What is predictable, though, is that the long-lasting variants will be ones that spread easier (more infectious).