Hi Ioannis,
I verified your calculation!
1. For an estimated mass of 1.8x10^54 kg the Schwarzschild radius calculates as 26.7x10^26 meters.
2. The estimated radius of the universe is 0.95x10^26 meters.
3. Hmm,I think this is too close to be a coincidence.
4. I chose mass and radius from a calculation I was making for estimating dark energy in the universe.
I did not cherry pick to make my numbers come out correct (at least not consciously :)
5. But if I cherry pick the mass of the universe from the estimates provided in the table I can get really close to the universe being a black hole. See Below.
I also tried another reasonable estimate for mass that was given in the table (1x10^53 kg). I stayed with the estimated radius of the universe as 10 billion light years (0.95x10^26 meters) because there was more agreement that this was correct. This gave a Schwarzschild radius of 1.48x10^26 meters. The actual estimated radius of the universe is still the 0.95x10^26 meters.
Yes this calculation is based upon best guess estimates. But my feeling is that your intuition (or did you know something) is correct, the universe taken as a whole is a black hole.
As far as I am concerned I am happy to contribute to your result. I am not sure how to do it but I think this result should be broadcast to the physics community, because I do not believe it has been suspected.
Give it a try!
Don L