Just to reinforce what others are saying, security isn't a binary yes or no thing, it's on a continuum that has tradeoffs with usability, and where you want to

Just to reinforce what others are saying, security isn't a binary yes or no thing, it's on a continuum that has tradeoffs with usability, and where you want to be on that continuum depends on risk. There are things you could do to be more secure than Signal, but they would also be more difficult to use, and many of those things aren't about Signal itself, but the hardware and networks it is on.