21 Days in Seville

You could try writing on the TripAdvisor forum, several of the people who reply regularly stay for several weeks at a time

I haven't spent that long in Seville, but had a pen pal who spent 3 months there on several occasions ...and I have looked at it for a winter long stay myself.
In my opinion, Seville is compact enough to stay anywhere. My 5 days was in an apartment in Old Town near the bull ring. A little challenging to get around the twisty streets the first day, but convenient to historical area and there is a small and there is a small mini bus to get you up near the Cathedral. Cathedral area is more 'local', near Parque de Maria Luisa gives daily walks in the park or along the river. Triana, across the bridge, is a whole other district to consider. My limited visit there is still a warm memory, wandering the riverside watching families enjoying g Sunday brunch, the young men still in their wedding tuxes just coming home (or to family brunch) after a night of revelry (and not all drunk or hung over).
I would look at any of these locations for my long stay, but TripAdvisor locals may have further suggestions.
Depending on your day trips, you'll want to consider how inconvenient it is to get to train or bus to make those trips. If I recall, there are 2 bus stations at some distance. I remember thinking i was arriving near Old Town from Granada, but it terminated some distance away, so I had to scramble to figure out local transit. In the end, it was relatively easy as it is on the trolley line.

Lucky you, Marty.

We stayed in Sevilla for a week in the Triana neighborhood in a VRBO apartment and loved it. Just over the bridge (two actually) it is an easy walk to the historic center and lots of great restaurants, bars, shopping, historic sites in Triana as well.

We've travelled to Seville on and off for the past 13 years and usually stay 6 weeks or more. We tend to visit  between November and March.  We combine Seville with other cities/towns.   

We've stayed in most neighborhoods. All have been excellent for various reasons. It depends how far you want to walk.  

We choose our accommodation nowadays based more on the actual street as opposed to the neighborhood. We know the quieter, quainter central streets. These can be a 5 min walk from main thoroughfares that are packed daily with tourists and locals, but you would never know you were amidst the chaos of Seville central. 
 
Bario Santa Cruz, Alfalfa, Arenal, La Alameda,  Magdalena,  all are excellent neighborhoods located an easy 5 to 15 min walk to most sites. All close to hundreds of tapas bars, restaurants etc. Supermarkets are everywhere. Mas, Cortes Ingles, Carrefours,  the usual suspects. 

If I had to pin it down I would probably choose El Arenal as one of our preferred neighborhoods.  But it depends what street you stay in. 

As an aside, cycling along the Guadaquivir river is truly spectacular,  if that's your thing. 

I can suggest some accommodation options with kitchens if this is of interest. 

Thanks, Rob_Chris. Based on your recommendation, I think I have found the perfect place.

But let me ask you this- I am undecided if I want to be in Seville during Easter week. So, suppose I had two alternatives- 3 weeks mid-March to early April, or 3 weeks from early April through Easter. Factors would be crowds, weather, etc. So, if you were in my place, had never seen Easter week in Spain, would you include Easter week, or not?

If you can get reasonable accommodations during Easter week, are religious or have an interest in the processions, then definitely visit during Easter week. There’s nothing equivalent to it in the U.S. A negative could be crowds if you don’t like them, but you’d probably most notice them when the processions are occurring.

Seville offers a lot during Easter Week, I am told...but it includes inconveniences. Parades go on for hours, so learn where the parade routes are....to consider avoiding them. Roads are closed, you can't cross the street to any hotel or restaurant, the crowds go on all night, so loud. Also, churches may be closed or items removed from view in preparation prior to the event. A friend discussed the Cathedral's preparations of the costumes needing to be cleaned and repaired the weeks prior then the dressing of the statues and preparations of the biers. Interesting if you can see them, but if not, these these items will be unavailable.
You may also find accommodation prices higher and will want to consider purchasing transport tickets earlier.
Also be aware of the Seville Feria (fair) which is possibly April 4-10, 2025 (see if you can find the town hall website to confirm. They usually announce it in the summer prior). Again, loud and party atmosphere....which will not be a US experience. Prices go up, but you have flamenco costumes, displays, new sights and sounds.
Perhaps there is a trip report on this site that spans Easter or Feria.

One last consideration...weather. Andalucia has been hard hit by global warming. As well as drought it has become more common to have temps over 100F by very early April. Check the historical weather for Seville for your exact dates to see what's been recorded for the past 5 years. Posted 'averages' no longer apply.

Wow, MariaF, what a great tip. I looked at the actual historical temperatures for Seville, and now I'm thinking of going in February! Thanks for telling me that!

Glad it helped. I check historical al weather for all my planning...first for weather comfort, then for packing.

Thank you Marty, pleased you've found a place to stay. 

In answer to your question. I personally would not include Easter for your first trip to Seville,  We've been in Seville during Santa Semana. The event is truly spectacular but can be overwhelming.  I can't begin to describe the crowds. The population multiplies, hotel costs soar. Many roads are closed. The weeks leading up to Santa Semana, when preparations are in full swing can be interesting.

The weekends in Seville are full on fiesta mode, no matter what time of year you visit. Lovely atmosphere, the Spanish certainly know how to enjoy themselves.

Every year we've been in Seville during Dec, Jan, Feb, Mar it has rained quite a lot.  If you are staying the usual 3 or 4 nights that tourists seem to generally prefer this could be bothersome. As you are there 21 nights the odd rainy day doesn't matter. There's always tomorrow to do things. February can be chilly in the mornings.  

We find late March,  April too hot in Seville nowadays.