From what you describe, it doesn't sound to me like your wife is fully in touch with her own needs, or maybe how to communicate them.
You say she's a very low maintenance person, with few wants or needs or desires. The thing is though - every human being has needs. The difference is whether you know what those are, or not.
She could be asexual, and is happy and content with sex when you initiate as she's very much in love with you. The descriptions of her mild enjoyment of sex, with no urgency, read to me a little like descriptions by asexual people who do sometimes have sex with they people they love. They get something out of it, but it's different to the physical raging lust that non-asexual people experience. So in this case maybe her needs are actually being met perfectly, but there's confusion between you, because your needs are very different, so you can't understand how she's happy.
Equally for whatever reason - she may not be asexual, but has fenced off part of her sexual self for some reason. But if somebody does not want to discuss that, then obviously there's nothing you can do, except make it clear that you're there for her to listen and be supportive if she ever wants to talk about it.
On a purely practical level though, I wonder if you could do something differently about what you describe below? You say it's a long winded process (definitely sounds more like work than sex!) so what would happen if you suggested you taking care of her using a vibrator instead of you having to give oral for such a long time? When the vibrator is doing it's thing, you could do some sensation play tracing feathers or ice gently along her body.
I feel like I'm starting to write erotic fiction tropes so going to stop here
Obviously it's only if you think it would be mutually enjoyable. I am not encouraging putting pressure on anyone else to have sex, or to experiment if they just don't want to. But my point is, sex is much more than just penis in vagina, or giving/ receiving oral.
"Just for clarity, I didn't say I don't like doing it, it's just that it's such a long winded process that is met with such little enthusiasm, that I wonder whether I should bother at all, or whether I should let her take some of the responsibility to help make it happen - hence the thread.
If it was requested, such as "I'd love you to go down on me, I'm dying to cum" or similar, I'd happily oblige for as long as it takes as that demonstrates hunger, passion and desire.
It's when I offer and it's casually accepted with mild enthusiasm, but with then precious little feedback or encouragement for ages and ages, where if I stop half way through, there's no protest or "Oh don't stop, I'm nearly there, please carry on...." indicating she's really into it and feeling horny and passionate about it, I just get disheartened and feel deflated, where I feel I should be feeling excited, needed and encouraged.
Imagine asking your husband if he'd like a blow job and he replied with "I don't mind" or "That'd be nice", then laid back, almost motionless, almost silent and left you to bob up and down for half an hour getting cramp in your jaw and neck ache, with little or no feedback/encouragement and it only very occasionally resulted in a climax - how would it make you feel?"