> Now, the thing is that you can’t just sequence a virus, do bioinformatics, synthesize mRNA and package it into a nano particle.
You can, and that's exactly what was done. However, out of the 10 or so candidates, they selected a few of the more promising ones to continue research.
They are fast to develop. However, there are other steps in the pipeline that aren't so fast.
> The mRNA that gets packaged needs to be modified (I believe with alternative bases) to achieve desired effect.
That's the easy part. The full mRNA sequence is not that large (https://berthub.eu/articles/posts/reverse-engineering-source...) and many of such substitutions can be done by algorithms. Now, maybe the modified protein won't fold right. That's not as easy (although, we can compute how it will fold now).
> If mRNA vaccine was as magical as it was marketed, we wouldn’t be getting 3-4(and soon 5?) doses of the same vaccine with the emergence of new variants.
If we were reckless and just wanted to inject someone with the latest update, this could be done in a matter of days. Human trials took months and people still complained that it was 'developed too quickly'. Then there's logistics.