While the question of whether Trump's actions would have resulted in felony charges is a matter of opinion (though the 1027 prosecutors who signed the letter you mentioned is strong evidence in the positive), it is a fact that Mueller specifically avoided making a judgement about indictable offenses and chose to stick to investigating and reporting the bare facts of the case.
Robert Mueller was operating under a memo issued by the Department of Justice's Office of Legal Council in 2000 which stated that, under their understanding of the law, a sitting President cannot be indicted or prosecuted while in office.
In 1973, the Department concluded that the indictment or criminal prosecution of a sitting President would impermissibly undermine the capacity of the executive branch to perform its constitutionally assigned functions. ... We believe that the conclusion reached by the Department in 1973 still represents the best interpretation of the Constitution
A SITTING PRESIDENT’S AMENABILITY TO INDICTMENT AND CRIMINAL PROSECUTION
For more discussion of this memo, see the question How is justice supposed to work for the president of the United States?.
Based on this memo, Mueller stated that he did not consider it his job to determine whether the president did or did not commit a crime:
“The special counsel’s office is part of the Department of Justice, and, by regulation, it was bound by that Department policy,” Mueller said. “Charging the president with a crime was therefore not an option we could consider.”
Mueller did not say explicitly that he would have accused Trump of a crime had it not been for the standing policy, but he reiterated that his investigation did not exonerate Trump.
“If we had confidence that the president clearly did not commit a crime, we would have said that,” Mueller said.
Mueller quoted by The Hill
The statement by former prosecutors that you mentioned also mentions this:
Each of us believes that the conduct of President Trump described in Special Counsel Robert Mueller’s report would, in the case of any other person not covered by the Office of Legal Counsel policy against indicting a sitting President, result in multiple felony charges for obstruction of justice.
The Mueller report describes several acts that satisfy all of the elements for an obstruction charge: conduct that obstructed or attempted to obstruct the truth-finding process, as to which the evidence of corrupt intent and connection to pending proceedings is overwhelming
STATEMENT BY FORMER FEDERAL PROSECUTORS
So, while I can't say for sure that Trump would have been charged had he not been president, I think the odds (of being charged, not necessarily convicted) are quite good. However under current legal guidance, as a sitting president he is currently immune to prosecution by the DOJ and impeachment is the only remedy for his (alleged) crimes.
When asked by AG Barr whether he would have indicted Trump if not for the OLC policy, Mueller did not say that he would have. Some have pointed to this as evidence that he specifically found Trump to not have met the bar for indictment. However, as uMdRupert pointed out, Mueller's office specifically addressed this point to shoot down this theory:
“The Special Counsel’s report and his statement today made clear that the office concluded it would not reach a determination – one way or the other – about whether the President committed a crime. There is no conflict between these statements,” they said.
DOJ, special counsel say there is 'no conflict' on Mueller, Barr statements about obstruction inquiry - The Hill