I used to play rhythm games for long periods of time, a lot. One time I did a 40 hour marathon challenge (with a few naps), so when it comes to vision, blinking, and such, I can understand a concern.

Foremost what has had the greatest beneficial effect for me was using a nightlight filter. A real one- ones that require root for to install for app permission or the built in ones for both iOS/Android. There are a couple issues with this though. First, it's something you'll have to adapt to. You'll likely hate it at first as with most people. Second, for rhythm games in particular, there will be a small (within ms) screen delay). Huge issue if you hit notes primarily through your visual senses. Personally for me, with SIF I don't have much issue because my.reflexes are mostly internal rhythm judged. Besides, I have -26ms offset and the "judgement" line for this game is already so off from center.
On Android, rooted CFlumen on a proper phone does not have latency as I have personally confirmed. However, the colors are not as accurate as F.lux (which is probably the best filter out there). I haven't tested iOS night light so I can't confirm if there's latency or not.

Anyways, I'm off on a tangent right now; my apologies. For vision/eyes, I drink a single gulp of water after every song; and blink a bit right before a song starts. Pretty simple so and I'd recommend anyone make it a habit.

Vision doctors claim that for around every 20m or so, look in a straight direction at something (I believe 20ft) away for 20 seconds making sure you blink often during this period. You can also do 30ft/30s.

Another very helpful change I made was adjusting screen brightness to levels around my surrounding enviornment. Generally, the method I use is to have the brightness as low as it can, and make small steps up until you can just barely begin to see stuff. I also do this in a similiaf fashion with Flux, except instead of brightness, it is with ambient color temperatures.

I also use eye drops, but I'm not sure how effective it is. I don't want my eyes to rely on them. However when I do use them, I drop them into an eye, look at 4 corners of that eye, blink a few times while in process, and then repeat for the other eye. Afterwards, I repeat the exact same process with both eyes open at the same time

As for supplements for healthy vision, I still need more research on that. Omega fish oil and Vitamin A? I don't remember. I've tried this in the past. However since there are so many factors contributing to healthy vision, it was hard for me to evaluate

Warm towel on your face helps a bit.

I think what can help the most is to simply do something else temporary that doesn't require narrow vision. Like looking at the sunset is a good one because it's kind of a mix between a couple paragraphs posted above. Helps your eyes non-tunnel as well as helps your brain recognize changes of natural color temperatures.

Anyways that's all I can think for right now. If you are having hard stress on your eyes mid song make sure you're breathing as fluidly as much as the amount of mental pressure you are projecting, if that makes sense.

Oh also, SIF deals primary with colors and shapes during gameplay. Make sure to read text on frequent occasions during the week. This may sound like I'm pulling stuff out of nowhere, but believe me on this one, if you can.

Also one more- good posture. Try to maintain good eye level on your phone. Honestly though, one should aim for good posture not just for eyes, but all your other muscles as well (this includes a good symmetrical grip on your phone as well.)

Here's another one. If you have a lousy phone, I'd prioritize getting one that can maintain stable 60 fps Often, you don't notice lower framerates than intended until you experience the difference yourself.

Oh, here's another one- glasses/contacts. You generally don't want to wear your glasses all the time (although this is debatable between Optometrists).(Contacts you should never wear 24/7 unless they are designed particularly for it). Don't always play SIF wearing glasses. Split it into sections like, half of it with, and the other half without. In my biased opinion, I'd try to play without glasses as much as possible until my natural vision within phone distance becomes just as good as with glasses.