I store my PLA and PETG in the bags they come in and have never done anything past that because I've never had an issue. I think people are far too concerned about filament being dry over a few filaments where it is actually important like Nylon.

I live in a really dry climate, so I don’t really notice issues unless I have a spool out on the printer for months, and even then dust is more of an issue than humidity.

To be safe, I bought a storage bin with a gasket on the lid that I store all of my other spools in, and that has seemed to work great.

Try running your dryer with the lid propped open a bit. Most Sunlu dryers don't have a way to get rid of the humid air pulled out of the filament.

I just put it in my dryer at 40C for a few hours with little regard to a target humidity. I also live in a dry climate where the ambient humidity is low, so maybe it's not enough in wetter climates, but this works for me.

23% is really just the bottom range of what your dryer will report. My dryer is 15%. Who knows what it really is below that, and whether those numbers are even accurate?

But after a day of drying at 50C you PLA will be bone dry regardless of what the dryer reports.

As close to zero as is functionally possible. There's no benefit in not shooting for that target, even if it is unlikely you will actually fully achieve it.

I store my rolls in a tote with a gasketed lid, with a generous amount of silica gel desiccant. Whatever spool is in use I feed directly out of a heated filament dryer.

A lot of people here commented "I do X and it works for me", but I do not think that is good advice. While it might work fine for that person, there are too many variables that are ignored. Ambient humidity, filament type, printer model, slicer settings, model geometry/details - all of this has an impact on the final print quality.

A more controlled environment removes variables and therefor makes the print result more predictable. Drying filament and storing it properly takes a bit of effort, but it is easy step towards better results.

You don't even need a dedicated dryer, just use your printers headbed, put 1-3 spools on it and a cardboard box with a few vent holes on top. Set the temperature according to the filament and let it run for 8 hours. Afterwards put the spools into a sealed container (4L cereal box works great), add some silica gel and your done. When it cools back down the relative humidity drops below 10% RH, which is so low that most hygrometers wont even measure it.

I'm casually printing PETG at 260°C, over 20mm³/s (about 300mm/s) and archive reproducible near perfect results with next to no stringing. With bare PLA drying may not matter, I've too little experience to give a definitive answer. If you have any composite filament (wood, carbon, sparkle, etc) you definitely should dry it anyway, because you do not know how much the filler changes the properties.

Oh and finally: I place new spools in containers with dry air (a tiny bit of silica gel in them) and measure the equalized humidity after a few days. Most spools were delivered with a humidity of 15-20% RH

Like everything in life: Depends.

A roll that will be gone within a few weeks? Don't care.

Long-term storage (exotics)? Vacuum packaging.

Regular but will be open for months? No clue. My Xiaomi temperature & hydrometer go to 0%. In a nutshell, just throw orange gel in a watertight box and replace it once it starts turning colourless/green.

20% should be perfectly fine. I can't get mine less than that without extreme effort, and it doesn't perform better at that percentage anyway.

I dry mine in a food dehydrator and print with it in one of the filament dryers to keep it dry during long prints. I live in a swamp though, so it's necessary for me to do this before almost every print.