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If you're getting flickers that's probably the best thing to do. This will cut your total power but you'll get a smoother signal. If it is visible then that is relatively low frequency but you might want to play around with it. The easiest thing to test (because you're more likely to have the parts accessible) is a pi-filter and then if that seems to be working you can either stick with that or use a better filter.
I’ve done multiple projects that use 20m+ of WS2812s. I deliver the main power in segments from a single large PSU and run 5v signal (despite being designed for 3.3v signal most will handle 5v fine and works better for longer strips). Running segments with connectors also makes it easier to swap out failed segments.
All WS2812B parts that I am aware of are specified for 5V signal and 5V power, not 3.3V.
Some brands won't even accept 3.3V input signals and require level shifting.
If you’re already doing segments with separate power injection you may as well split it entirely.
I don't think in OP's case a controller such as the Advatek ones would be attractive, they are quite expensive but perhaps some WLED-flashed boards being centrally coordinated could be a last resort given it would increase complexity and potential brittleness to the setup.
But it would also be a good idea to measure voltage drop when it's powered on, and compensate accordingly, if your PSU allows for it.
If you need NEC class 2, both Lutron and ERP make UL listed 24V class 2 supplies. You can calculate what length you can drive at 4A (96W) and drive that much. The Lutron one dims and the ERP does not, but you can chain an eldoLED or other “dimmer” between the ERP supply and the strip with excellent results. There is no perceptible flicker with either of these, and IIRC both Lutron and eldoLED even claim IEEE1789 compliance. (At least the Lutron one does leave sort of perceptible spots when dimmed when you saccade perpendicular to it, and you need to go to rather high frequency to avoid this. Of course, this is a nonissue at 100% duty cycle.). I have used both of these with strips from different power supplies butted up to each other with no perceptible brightness mismatch.
48V strips are a thing.
If you don’t need NEC class 2, you could try a much larger DC supply. You might consider circuit breakers to protect individual segments and wires. DIN-rail thermal-magnetic breakers rated for more than 24VDC are cheap, but they will buzz horribly if you apply PWM to them.
You’d be on your own in regard to regulation, but I bet most “120V driverless” strips work just fine at 120 times sqrt(2) DC volts, and they won’t flicker. You will get more than the design output and more heat, but no flicker. Test first and take appropriate care for safety.
Don’t use “driverless” strips with sine wave input. The 120Hz flicker is horrible.
Some LED strips contain internal constant current limiters that will give a roughly constant output across a (small) range of voltages. This may help.
If you supply ground from one end, and +24V (or whatever) from the other end, brightness will be equal throughout the strip :) But it would also be a good idea to measure voltage drop when it's powered on, and compensate accordingly, if your PSU allows for it.
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That being said, 20-50m is a really long run even with 24V LEDs. Even using this trick, you'll run into significant voltage drop and heat in the LED strip's copper traces since they're only so thick. There's a reason why manufacturers specify a maximum length. I would check the datasheet and split the strip into multiple segments depending on this value. Maybe there are some LED strips designed for this use-case, with an even higher voltage and/or thicker traces.
You can't really avoid power injection downstream. I'm not sure if you're using 24v LEDs based on your ask (you said rails). But even with 24v strips at 20m you'll need 4A edge injection with 720 LED/m. This is literally one of the most simplistic approaches without being under powered.
[0] https://quinled.info/the-ultimate-led-strip-power-injection-...
If you supply ground from one end, and +24V (or whatever) from the other end, brightness will be equal throughout the strip :) But it would also be a good idea to measure voltage drop when it's powered on, and compensate accordingly, if your PSU allows for it.