The Ebro Delta is fantastic. We've visited many times. Presumably you're taking a car and I'm guessing going for an overnight. In which case, ensure that you include the following sites - all of which, I'm sure, are eBird Hotspots:
Lo Golero
Riet Vell (hide overlooking a pond)
Playa del Trabucador (it took us several years before I summoned up the courage to drive so far on a sand-spit but the 'road' is fine. Probably wouldn't risk it in the rain

Mirador de Migjorn (called Mirador David Bigas on the eBird hotspot map) - worth going the extra couple of hundred metres to the beach here as well, especially if you haven't already seen Audouin's Gull, although they are possible virtually everywhere.
We also like the area around Finca Bombita, reached on the minor road north-west from the T-340 at the Parc Deltaventur roundabout. We've only been here in the autumn (Bluethroat, Stone Curlew, waders, terns, Thekla Lark in 2023) but it might be good in the spring too and it's very quiet in terms of traffic so you can drive and bird reasonably safely and park anywhere convenient for a wander around.
We usually stay in the Delta Hotel, just outside Deltebre. I think they have breeding Scops Owl in the grounds, as well as Penduline Tit and in September/October at least, the trees in the grounds are a roosting site for Black-crowned Night-Heron. If the fields are in the right state, kingfishers, egrets, ibis and more can be observed through the windows over breakfast and the set evening menu is generally quite good, often with locally sourced foods
We don't really bother with the Costa Brava. We're not much interested in beach resorts, even the lower key ones that you should find there and the birds are probably similar to those on the delta. If we were to stay in the area, I would imagine that the Roses/Empuriabrava area would be the main draw for us, because of the nearby Parc Natural dels Aiguamolls de l'Empordà. The Pyrenees certainly have some good birds that can't be found in the rest of the region (Wallcreeper, Bearded and Cinereous Vultures, Citril Finch, Alpine Accentor). I don't know how easy Capercaillie is but I think they're possible. Stephen, referenced above, will know.
Our preference is for the southern half of Catalunya, but that's in part because of the culture. Many of the best festivals are in that part of the region and we rather like the unique landscapes of the delta, the limestone mountains of the Priorat region and the dry-land birding closer to Lleida. However, my partner is also a birder with a life-list well over 2000, so I don't have the 'problem' of having to find a balance. We tend to find it naturally. We're probably going to spend a few days around Teruel in June and we're already talking about perhaps doing mornings of serious birding then exploring the city in the p.m. I do feel that there is a wider range of species in the south, as compared to the Costa Brava, but as mentioned, the high altitude specialists are only in the Pyrenees. There is never enough time.