If we are planning to dive into bike fit instead of the brake reach issue here), we should move over to my actual bike fit thread.
But in short, based on your height, inseam, bike frame size and stem size, there is something interesting about your fit.
As most fitters about an 80mm stem on a 58cm bike and you will likely see raised eyebrows. It’s an uncommon combination at the very least. May well be right for you, but it is unique.
If possible, let’s move to the other thread and add some pics of you on the bike leaning on a wall or in a trainer. Side, front and rear pics for starters. And a side pic of the bike only for reference.
And include any issues you have or want reviewed other than the saddle slip issue already mentioned.