When bonding with my bird, I always talked about what I was doing around the house before I did it and as I was doing it. E.G., I am going to use the big vacuum, or I am going to stand up...or I am going to take out the trash. This helped her learn vocabulary and it also built her anticipatory skills. If you leave the room but are nearby, you can talk to your bird to let him/her know you are still near---this can sometimes prevent screaming before it starts. That having been said, I never ever respond to screaming once it starts. As your bird becomes more comfortable, he/she may demand your presence via screaming-- you do not want to come into the room in response to these vocalizations-- you also shouldn't call back once they start (in my opinion). You should ignore screaming and withdraw attention/eye-contact etc. I leave the room or turn around. When the screaming stops for a solid 15 seconds (which can sometimes take a long time), return your attention and praise for being quiet. Also, if your bird makes a sound that you like in an attempt to get your attention, you can praise/return as a result of that sound and it will often become a replacement for screaming. My bird says "I love you big bird" to get my attention these days. But I digress....
More on bonding- If You can try feeding your bird food from your hand if he/she will take it without hesitation, but if your bird seems hesitant, just place it in the bowl. I also let mine come in and out as she pleased as long as I was home (excluding days before work when she wouldn't yet step-up and I had a tight time-frame). You can try playing different types of music and see if he/she reacts more favorably to one than the other. Read nearby, sing songs if you want, do paperwork nearby etc...Just live your life and show that you are not a threat. Don't rush contact- the bird should lead the way on that- especially if the bird already came to you knowing how to step up but is choosing not to.
In terms of the harness training and how much longer you should wait, it depends-- how is he/she doing? Does he/she step up consistently without biting? How long have you had him/her?
In terms of showering, you could get a squirt bottle and spray yourself with it a bunch near him/her and just see the reaction...If a spray bottle was ever used as a punishment (bad...) then it will likely be obvious based on the bird's reaction. In that case, spraying the bird would be a bad idea. You will want to make sure your house is quite warm if you wet down the bird and make sure that you never do it at night---only in the morning/afternoon and when no drafts are nearby. Think about how 78 degrees can feel cold when you get out of the shower and then imagine that you were unable to regulate your body temperature as well as humans are able. My bird likes to play in a glass pyrex pie plate at the bottom of her cage (filled with some cool or luke-warm water). If you do this, make sure you clean the grate etc (because you don't want him/her walking through poop and them putting her feet in the water and drinking it..). Also, keep an eye on it to ensure that he/she isn't pooping in the dish or slamming into the glass hard enough to chip it. My cockatoo has never had an issue, but it's best to be cautious. Afterwards, you will want to dry the interior of the cage (under the grate etc) to prevent mold and bacterial issues.
You don't want to start anything "scary" until you guys have a solid bond. I wouldn't start with the harness until your bird has been with your for at least a solid number of months and is stepping up without issues...For mine, I wouldn't have dared try before 4 months (and that would likely be too early in many cases). Maybe others will have a different opinion, but since this is an adult bird, you don't want to rush it and scare him/her. You could hang the harness nearby so that he/she gets used to the look of it, but I wouldn't try to put it on yet...In order to do so, you will have to be able to touch your bird's head etc without getting bitten. Has he/she worn a harness before?
It is all going to come down to how solid your relationship is. If you scare him/her, you will damage trust temporarily, which is why a solid foundation is needed in order to prevent "debt" in the trust department.