With something that needs a lot of water, I would add a lot of vermiculite to the soil. It holds three or four times its volume in water and makes it available to plant roots. Half vermiculite is not too much, but whatever you can add will help. I buy vermiculite from Amazon, because stores often only offer smaller bags at high prices. Amazon has four cubic feet for $40. That's a lot of vermiculite. It's very light and fluffy. You want the course grade. You may also find large bags locally at swimming pool supplies. It's all the same stuff. It's just volcanic material that fluffed as it dried. Nothing harmful about vermiculite currently being mined. I use 100% vermiculite to root cuttings. If it does what you want, you will have to do it again about every two years.
I wold normally look to mulch for water control, but I also don't want to have to deal with it in an annual bed. I think vermiculite is the thing.
Perlite is also used, but I don't like it as well and not at all for your problem. For one thing, the perlite itself doesn't hold so much water. It just presents some surface. And it is more obvious in the soil. It looks like, but is not, plastic foam pellets. And it floats and moves around the soil. Perlite is also a volcanic product that's popped into expansion by heating. Perlite may be better for plants that can easily get too much water, because it helps create some air space. Perlite costs about the same by volume as vermiculite. I do use it, mainly in containers that tend to get compacted.
If you use either heavily, consider adding nutrients to the soil as neither of these provide them. Good time add compost, anyway, since you're already digging. May need to anyway, if it hasn't been done in a long time or forever.
