I have always enjoyed anal sex a lot, but had a few relationships where it was barely there and usually almost non-existent. When my fiance and I first got together, we fucked in mouth, pussy, and ass. As the relationship grew over time, I found that we were more often than not ignoring the inclusion of anal in our sex lives. This bothered me, so we started talking about it more.
To give a little background on her: She was with a partner for several years before she got with me. She was anal-only with her ex for the first two years or so of their relationship. The reasons for this are less than noble, since it involved her ex refusing to ever fuck her pussy because he thought she was ruined goods. Eventually after they broke up and got back together, he finally offered her vaginal sex which she gladly accepted. At this point, that was the very first time he had put his dick all the way up in her pussy. Based on her explanation, the most he ever gave her pussy before that date was just barely inserting the tip of his cock into her but then pulling back out just to slide the entire length into her asshole instead. Apparently the rest of her life with him (while their relationship persisted) and three kids later, they mixed it up with a mixture of oral, pussy, and occasional anal.
When I first got together with my fiance, she explained that she LOVES anal sex and wanted to have it with me. The first types of sex we had a lot WAS anal, until it eventually dropped off.
Lately, we have both been talking about taking our D/s lifestyle "to the next level." She is very devoted to me, as I am to her. She is a fantastic submissive and works to keep the house clean, the clothes washed, the food prepared, and my cock satisfied and honestly gets upset every time I lift a finger to help myself. She says she gets hurt when I do anything for myself since "that's her job", according to her.

I want to use this opportunity to push for more anal sex now. For the past few nights, I have randomly selected which days I wanted to plow into her ass, and did so. It was pretty uncomfortable for her, but that was part of what BOTH of us got out of the experience as well - it enhanced the experience for us. I told her that my goal is to train her ass to where it can accept my cock on short notice, without a lot of prep. She explained she knew what this was like since her ex had no issues entering her ass after a while of fucking anal-only. She also seemed receptive.
So finally, after ALL that background, let me get to my question(s). My fiance doesn't like anal sex that much. She likes it when it pleases me, and she enjoys submission to me and getting fucked hard in the ass makes her feel VERY submissive to me... and she likes that. But it is hard to get her started on anal most of the time because she seems reluctant. Many times once I get her going, though, and fucking her good - she gets into it and even begins fucking back at me. My question is this: Are there any tips I can get from other male dominants out there (or their female submissives) on the best ways to use D/s to train a submissive for lots and lots of anal sex, especially when it isn't really "their thing"... but they are willing to submit to you in any way you desire. I don't want to abuse the power I have over her, and I don't want to push her TOO much. For example, I think if I told her that she was going to be anal-only from now on, she would break down in tears and be very upset. I don't want that - I want enthusiasm. I am sure many experienced BDSM practitioners can tell me if they have good tips for how to train a sub to enjoy anal (especially with a D/s angle). I am also VERY curious about vaginal chastity, clit shields (she already has a clit hood piercing), and pussy-lip piercings to make the vaginal chastity more fun and kinky. I am wondering if small periods of time of anal-only would be a good option. Is anal-only based on D/s reasons even realistic? Please, share any thoughts and advice you have. I appreciate it a lot.