Personally, I think it’s great that you want to share this with your partner. It is hard to know how to make specific recommendations for how to frame this for a woman I don’t know personally. ‘Different strokes for different folks,’ and all that.
Still, your brief introduction to the problem sounds like asking her is not the way to do it. And framing around “time” doesn’t sound that compelling to me, and I doubt it will to her either given that she is not kindly disposed to this sort of activity anyhow. Have you instead considered simply telling her that she has been completely clear with you about this, but that you are going to do it anyway? Then, be really clear to her that you could do it privately, but you want her support — doing this is just that important to you, and she is important to you too, and so you’re telling her even though its not something that apparently inspires her (but that if it works, she just might find it’s a fun thing). Ultimately though, she can be a good wife and supportive partner, or turn a small thing that is important to you and has very little impact on her day-to-day life into an issue.
The basic idea is thus to make the conversation emotionally compelling and to put the emotion-work burden on to her. Presumably she loves and supports you, so set her up to do that. The burden should not be on you, but framing in terms of “but I can do it in 30min or less!” does put it on you and gives control to her. If you set it up right, you should be able to confide in her and turn this conversation into one that brings you both closer together and ends with your dick in your OK grip.
A secondary thought: Does your wife kegel? You might consider discussion ‘sexual fitness’ more generally and the fact that there are things both of you can do to improve your sex life together. After all, you’re most interested in girth work and she can no doubt tighten up more quickly than you can expand. I was quite clever with my wife when we began doing PE and I pretended I would only do it if she would kegel (I was excited to see that she was going to support me but I played it cool). I got her some cute little weights (ami by je joue) to use and I think it turns her one when she works out while I’m doing PE. I’m white and she’s black so we’ve joked that we’re going to grow me a black dick and her an Asian vagina.
And why have the partner involved? Easy — mine has told me a few times that “you’re hitting new places and it fills really good,” and that’s more motivating than a ruler. I must confess that pulling on dick and thinking about it all the time, and being more positive about my little guy, has ratcheted up my sexual interest as well.
I wouldn’t bother with this sort of thing with a girlfriend — to much relational uncertainty.