You wrote:
"I'm not sure honestly how to approach him about this - as with a number of other things I've tried to bring up in the past few years, he gets extremely defensive whenever I try to suggest something."
Can you see how that statement is an even bigger, and worse, red flag that there are problems, than the whole homework issue? The homework problems are another symptom of a much larger issue your family -- your whole family -- needs to work on pronto.
If you are leery of talking to your own husband about something as basic as your child's homework; if you are finding that every suggestion you make results in defensiveness that's "extreme" (as you yourself put it) -- you and he need couples therapy and/or he needs individual therapy. Don't put it off or wait. Find out why he's hair-trigger defensive about his choices and why he's so overly tough on a first grader.
She is going to remember this. She is going to remember that daddy expects her to be perfect, and over time, she might think that if she isn't perfect (at school, at anything else too) she will make daddy mad and maybe....daddy won't love her as much. He can tell her all day and all night that he loves her but if he yells at her for not doing her work HIS way, she will take her cues from the yelling and the ridiculous expectations he shows -- not from his declarations of love. Do you want that for her? For their relationship?
She is also going to learn not to upset daddy because she sees that mommy doesn't want to upset daddy either. Your daughter probably picks up, much more than you realize, on the fact that you avoid talking honestly with dad. She will think that's how adults relate to each other. Do you want to model that for her?
Please tell him -- at a time when he is calm and has not just been fussing at her -- that you are worried not just about homework but about the larger picture of your marriage. He may not see that, but I hope you can see it. You WILL get a defensive, angry response from him because, well, telling him you both need therapy is going to be one of those suggestions that may generate his "extreme defensiveness" but you have to be ready to deal with that.
Did the defensiveness over his choices and actions start around the time he became a stay-at-home parent two years ago? That's something to think about. Is it possible that he's so defensive because he fears you are telling him, any time you suggest something, that he's not doing a good job at being stay at home dad? Was he a breadwinner before staying home? Are you certain he wants to be at home like this? I'm not saying it's a bad idea; I'm just saying think about whether it's possible the defensiveness comes from fears and misgivings about what might have been a huge change in his role in your family. This really, really needs to be unpacked with a therapist, not hashed out at home in ways that will make him feel even more defensive.
Script out how you plan to tell him that you both need a couples therapist (I would go ahead and have an appointment set up so he doesn't have to wait weeks while you find one). I would be sure to tell him that there is a larger picture here and it shows a wife who is afraid to communicate with her husband, and a father who has expectations that his wife and daughter don't understand.
And yes, you absolutely must become the homework parent. Pitch it as "This would give you and Sally some breathing space. You do so much for and with her -- you are great at (pickups, getting her places, getting her ready, playing with her, whatever). I'm glad to give you this break. You don't have to take it all on yourself." Do that separately from the therapist discussion -- I'd do the homework changeover first, then tell him about the need for therapy.
You really don't want a little girl growing up like this. I promise you -- if she learns to hate homework (and possilbly school, because it creates homework) this early, it's very hard to get her back on track liking it.