I apologize in advance for a kind of long reply but you have some good questions and the answers are important!
Although a plug with the narrow neck required for constant wear does not keep your hole dilated much at all, I believe that if you are patient you will eventually be able to relax your sphincters more easily whenever you wish to insert or remove toys/body parts/etc.
I really don't know what the time limit is on how much time you can spend wearing a plug that keeps your anus substantially dilated. If you wore one of these long enough that you stayed very loose even after taking it out, you could still close yourself back up before too long by doing kegels to recover. It's just not something to do every day and all the time. I actually recommend kegel and reverse kegel exercises under any circumstances, and they will certainly help with anal training if you practice them for a few minutes a day both with and without anything inside you. Also do a reverse kegel to relax as you insert or remove anything.
You could try some anal toys with wider necks for dilation, and then set some time aside each day for working them in and out. Over a period of many days or weeks, you can move on to using the larger sizes after beginning your sessions with a warm up using the smaller sizes.
These seem like a good value for a set of dilators:
They also have a 1 inch diameter stainless steel probe that seems perfect for warming up with before larger insertions:
You could also use a set of gradually increasing diameters of these for dilation sessions if you want something really luxurious and made out of solid pyrex glass:
As with any other muscle training, it helps to take a day or two off every week or so. You may also want to try the njoy and luxotiq plugs I mentioned earlier, since glass or stainless steel or aluminum are smoother and create less friction inside you than a rubber type plug tends to do after leaving it in for a while. Another benefit of these materials is that your body can't absorb any chemicals from glass or stainless steel or aluminum toys.
And of course, always use plenty of lube. You may want to do some research on organic coconut oil for this. I have recently heard from others that it is a great lube.
As always, I have no affiliation with any company I mentioned. Use the products responsibly!