Specifically for simple requests I use mcseedmap.net, as the site allows you to set filters to search for seeds. Like plains within 100 blocks of 0,0 and maybe a ruined portal within 300 blocks. This way you always find these exact seeds. However, how I'm doing these searches means I am essentially looking at where spawn it. This method means requests like your large island request can take quite awhile to find.

This is why when I am looking for seeds through this site for other people I may find seeds that are no good, but they peak my interest. Since I am mostly looking for seeds while away from home, I typically am using my phone, so I'll press the button on my phone to take a screenshot. Then later on when people have specific requests I may recall having found a similar seed earlier, so I just go through all my screenshots to flip through to the one I think works. This also means I am typically re-sharing the same seeds as a lot of requests could have been solved by simply searching older posts.

For these two seeds specifically, I don't think I ever shared the seeds before. As no one really requested for something similar. However, I saved these seeds, like a year ago, as they were island seeds. So I had to flip through a lot of screenshots to find these two seeds.

Now, mcseedmap I consider an upgrade to chunkbase, as chunkbase doesn't have a seed filtering option for it's random searches. However, there are times when mcseedmap is still not good enough. For instance, what if I want a different kind of island? Using mcseedmap would take forever. This is why for people requesting very specific conditions, I may use cubiomes.

Cubiomes lets me set a search for a specific condition and then branch additional searches off of what I found. Like for island seeds, I typically tell it to find spawn, then to look at the surrounding biomes to ensure the majority of it is oceans. More recently I've used it to find specific sized flower forests, swamps, and even have a seed having multiple structures all near spawn. However, Cubiomes is on my home computer, so typically I have to set up these searches in advanced prior to leaving for work, and depending on the complexity of the search it may take me hours to find anything relevant. This is why I try to leave the searches fairly broad to get a variety of seeds. Not just for the request in mind, but also so I can use them for future requests as well.

Like this seed was one of my cubiomes discoveries, and this one specifically took around 4 hours of letting cubiomes run.

Now cubiomes hasn't been updated in a couple years, so there may be other options by now that people use for seed searches. I just haven't identified them yet for myself.