There is no question that staying at the Epcot resorts is quite special. DW and I stayed there many times at the Boardwalk and loved being able to wander over to Epcot in the evenings. Especially during one of Epcot's festivals. We really enjoy visiting Epcot during Flower and Garden. The last few years we have been there on opening day. Which is a week later than last year. Perhaps to give the flowers a better chance at warmer temperatures. A few years ago I recall a very chilly F&G opening day.
As you have discovered, the Epcot resorts have gotten quite pricey. Especially during busy times. But the location really makes it a worthwhile splurge, especially for adults.
The calendar on Your First Visit shows the first and last week of April as lower crowd periods. Though crowds have become less... predictable... in recent years.
In addition to Epcot I suspect your mother might also appreciate the AK. DW and I spend a lot of time visiting there. The Lion King show is the best on site. The Safari and the various animal trails are very well done. DW and I find ITTBAB a fun attraction. We usually spend a morning wandering around AK. You should also wander around Pandora and take the boat ride Navi River. Incredible theming! Some good food can be found at AK as well.
And of course the Magic Kingdom is "classic Disney". Just the walk down Main Street is worth the visit. There are plenty of attractions like Haunted Mansion, Peter Pan, Little Mermaid, Small World, Philharmagic, TiKi Room, Riverboat, Steam Train, and CoP that are gentle and well themed. If you add a bit of silliness like the Muppet Show, Jungle Cruise, and Country Bear Jamboree there is a lot to enjoy at the MK that is gentle and not a "wild roller coaster" ride.
I might suggest two days at Epcot, a day at MK and perhaps a 2/3 day at AK.
If you end up at an Epcot resort you might consider splurging on park hopper tickets so you can wander over to Epcot any time you want.
Wishing you a
visit!