As an experimentalist, will it be possible to be a good theoretical researcher through self study?
Actually, pioneers in a field have no other option but to learn from self study. So, yes, it is possible and there are others before you.
In fact, for non-pioneers it is easier than in the past since there are now resources that didn't exist in the past. You have named some of them. You no longer have to depend on yourself and obscure papers that are probably misleading.
I assume that many of us here changed fields after their doctorate as I did.
But, a circle of collaborators or other contacts can be a big help in this so that you get some guidance. This is fairly easy if you hold an academic position at a place with, in your case, theoreticians.