Bruc, Hvar and Korcula island

I would stay on one island and do day trips. We loved Korçula..

You'll want to check schedules carefully. See Krilo and Jadrolinija websites. It can be difficult to do day trips from one island, but it is not impossible. Some day trips mioghtbe better as paid excursions if the schedule does not allow for day trips. I would not necessarily do one night in each, but it depends on how you like to travel. I would probably choose one and not worry about what I might miss.
April will be quiet and likely no swimming, but I would not avoid the islands in April.

We also liked Korcula, but late October meant shops had limited hours and some restaurants were closed for the season, so keep that in mind as April may also be out-of-season. Hvar was pretty quiet - it's known as a beach resort town, so I would not expect much to be open if mid-April is not quite the start of the season. In late October, nice to have no crowds but like Korcula, options for food, shops, markets etc. were also limited. We did go to Tomic Winery (outside of Hvar town in Jelsa so you will need transport), which was fantastic and we highly recommend a tasting if you like wine. You will want to look at ferry schedules in April to determine what makes sense - they likely will not run as often as "high season."

Mid April on the islands means not much will be open and the ferries will still be on their winter timetable. I would stick to the mainland and do one day trip to Brac or Hvar.

The water will be cold in April and villages will not be fully open. The tourist industry has attempted to expand the "shoulders" of the season, into April and October, but the weather is not as nice.

@Steven - We are ok with water not being warm as we don't play to swim.
But otherwise,
1. Is it better to stay in one island (Hvar/Korcula/Bruc) and do day trips?
or
2. Is it better to stay in Split and do day trips? (as island may be deserted in April)

I visited Hvar, Brac, and Vis last month. I spent the first four nights in Split. It's really easy to day trip to various islands from there without a lot of planning. (Though if you plan to visit more than one town on an island, be sure to check the off-season bus schedules. I sadly was not able to visit Stari Grad on Hvar because I didn't pay attention to the bus schedule and realized too late I had timed it poorly to be able to visit Stari Grad from Hvar Town. The TI in Hvar Town didn't have the winter bus schedules yet and gave me the wrong one.)

I didn't plan to spend any nights on an island, but I was so taken with Vis that I came back the next day and spent two nights there. The island was pretty dead in October, but that didn't matter. There were still restaurants open and the weather was nice. (And I was able to swim, but you won't in April, and I know you don't care about that.)

Imo Stari grad in Hvar is just OK I know Hvar town has this “party town” reputation but it’s so much more than that. And it’s also has wayyy more restaurants, bars, points of interest than stari grad imo

You’re going to be spending a lot of time just driving and on ferries with your plan. Imo I’d much rather just stay In one place than try to get to them all.

We stayed 6 nights in Hvar and we could have stayed longer. We spent whole days at the beaches in late October.