After 30+ years of marriage, and now being middle aged and noticing what a lot of women do, I would suggest to find something she loves now within the framework of what is classic/timeless and the budget. We all change our tastes over time (although hopefully not in partners!) and going classic now will likely prevent wanting to change out the ring in the future. The other option is to go antique if she's into that, as antique doesn't go out of style (antique -- not vintage or retro) for the person who loves it. I wear an antique cushion cut diamond in an antique halo mounting. It's nothing like current cushion diamonds or current halos. It's large, and VS2, but of lower color, which brought the price down. It was an anniversary gift.
But I don't wear an e-ring everyday, and I'd stopped well before the pandemic. It's not practical to wear it when working with bread dough, gardening, making beds, or cleaning house. Among friends my age and older, I've noticed many choose to wear a simple wedding band. My grandmother stopped wearing her engagement ring (passed down to my sister) a decade or two before she died, as did most of her friends. I also know nurses who don't often wear their e-rings, and mothers who don't wear them for fear of catching on a little one's delicate skin.
If I was buying a diamond engagement ring right now, I would choose a round diamond in a 6 prong solitaire mounting (but not set too high), either platinum or yellow gold (at least 18k). I would go as big as budget would allow, but only for an excellently cut, high clarity, colorless or near colorless diamond. I'd likely go no lower than VS2 or a high SI1 in clarity, and no lower than G in color. I'd look for a stone that falls just under a desired weight, such as something in the .95 - .99 range, or 1.45 - 1.49, or 1.95 - 1.99, etc. An emerald cut is also classic, but for an EC to be gorgeous you need higher quality and color -- probably no lower than F/VVS2. Emerald cuts don't hide color or inclusions well. And emerald cuts cost more than rounds.
I'm of the opinion that 1.25 ideal cut G/VS2 diamond in a classic 6 prong solitaire mounting is an engagement ring most women who want a diamond engagement ring would be thrilled with and could wear their entire lives. A 7-8mm stone is a size that looks great on most women's hands. I would be far more likely to pair it with a diamond eternity band than to turn to a halo for extra sparkle.
And I would search high and low and likely buy used if I could. Diamonds are like cars, losing value as soon as you buy them. And poor quality diamonds suffer even more. Don't buy without a GIA certificate (not the same as being appraised by a GIA certified appraiser) or an AGS certificate, but absolutely not EGL or any other grading lab, and for certain, not based on some mall jeweler's evaluation. Do not buy a diamond that isn't certified. Ask to see the certificate and an ASET. Maybe sure the diamond is laser inscribed with its certificate number.
My original engagement ring was a .24 round diamond set in a yellow gold solitaire. We paid too much because we were too young and inexperienced to even know we could negotiate with the jeweler, but we at least got lucky with the color and clarity -- it's a happy sparkly stone. And we didn't pay interest on it. I wore that ring for more than two decades, never begrudging its size because it signified the years we had together, and it never held us back from any financial goals. The big ring came when buying it wouldn't endanger any financial plans, and it wasn't even my idea. I love it, but I'm perfectly happy wearing a plain wedding band now. We're probably lucky we got engaged young and with no idea of cultural expectations about engagement rings -- had it been in our mid-to-late 20s, we probably would have gone into debt to get something bigger.