1.14, as far as I'm aware (dunno if it's been fixed yet, or if it has if the fix is actually working), has a bug wherein if a pack lands near a chunk border and a subsequent member attempt from that pack travels across the chunk boundary it cancels all the remaining spawns. You can fix this by limiting your platform to fit entirely within the chunk in such a way as to have an air gap along the edge of the chunk (because of a change in 1.13 to how mob spawning worked, spawns will no longer travel across this air gap at all). It's possible, of course, that 1.14 changes this too or that another bug interferes with this.
Additional bugs have allowed the 70-mob hostile cap to be exceeded (dunno if this one's been fixed either), causing a passive-cap style of problem wherein mobs are building up elsewhere and never allowed to get back close to the cap so more can spawn somewhere else. Because of the core way that the game distributes spawn attempts (see next paragraph), there's a potential for like 750-1000 hostile mobs being rolled every single gametick (20 ticks = 1 second) even though the mob cap itself is set to just 70. This is designed to keep the mobs evenly distributed within the world, instead of instantly bombarding the player with an overwhelming force or creating large areas of no mobs at all.
The core part of mob spawning that has basically never changed once since implemented is that a spawn attempt is made once per tick per loaded chunk. A pack spawn is considered 1 attempt for this purpose, so all members of the pack will spawn (or fail trying) rather than taking multiple passes to fill the pack. The horizontal coordinate of this attempt is chosen completely at random, but it's limited to the spawnable blocks in the chunk so any column that is nothing but air, glass, or bedrock will be ignored.
Things you are doing very wrongly (they are severely impacting your spawn rates):
1)building at y=200.
2)filling in the chunk below the grinder (with spawnable blocks)
In 1.12 and below, y-level dictated spawn frequency. Regardless of any other factors, the higher you placed a block the less likely it was to have a spawn attempt land on it. The breakdown of this means that y=6 has the best chances, with y=63 being roughly half that chance and y=200 being roughly one-quarter of that chance. This had zero impact on whether a mob could actually spawn there (lighting, block type, liquid present, enough space to fit the mob, etc), all this affected was the block the spawn mechanics randomly chose to do its testing on.
In 1.13 and above, the spawn weighting was changed to be a weighted average of the number of blocks in the column that could support a spawn attempt (glass, bedrock, and air were completely removed from the equation for this purpose). The number of spawnable blocks in the column would decrease the likelihood of a spawn on a specific block in that column at a more equal rate than in 1.12 (while no longer as important, "lower is better" is still a rule of thumb). What this means is that in a situation where your platform is the only spawnable block in the column it completely doesn't matter at what y-level it's at, it will spawn mobs extremely fast. Of course, this wasn't the only thing changed regarding spawns in 1.13 and 1.14 so there will be other factors that can slow the spawns back down to reasonable levels, chief among them being that mobs cannot spawn where other mobs are currently existing and blocks that formerly could not spawn mobs (water and leaves) are now included--which means oceans are very bad places to build mobgrinders at now unless you specifically want to have a never-ending supply of Drowned, and you can no longer safely decorate your grinder with leaves (even though only parrots and ocelots can spawn on leaves in jungle biomes, the game doesn't account for these factors and simply allows any spawn attempts anywhere on leaf blocks...which outside of the above two cases will automatically fail).
3)inside farm is darkness ofc
4)im 30 blocks far away from farm
I'm making the assumption that you're using what is known as a passive-flush design. These are grinders that allow the mob to walk off a platform, usually into a water stream that carries the mob to a larger hole that drops them into a kill chamber where they either splat immediately or are reduced to near-death so xp can be collected. There is nothing inefficient about these as far as spawn rate goes, however, there are some factors that CAN reduce efficiency:
Firstly, mobs in the 24 to 32-block range are in an active-AI mode, which allows them to wander around or detect players/mobs and react accordingly. When 33 blocks or beyond, they move into a passive-AI state which disables most if not all of these actions and they basically just sit there waiting to despawn. Mobs will only natural-spawn beginning at 24 blocks away from the player, so it's entirely impossible to keep all of a 1-chunk platform inside the active-AI range unless you're above or below it.
Secondly, mobs can choose to not walk off ledges when they can see it's a ledge. It's not really a big deal for small drops, but if the drop is more than 3 blocks long they won't walk off at all. Use trapdoors to fool them into thinking there's a solid block where the drop is. Mobs can also pathfind through walls they can't physically get through (why Mojang doesn't fix this I dunno). While this is mainly aimed at cage spawner designs where a platform is not needed for spawning (gnembon specifically discusses this in his two simple blaze farms videos from his Fun Farms series), I suppose one could use this to force mobs to go to a particular spot, therefore allowing you to drop them into a hole immediately instead of waiting on them to find it (one would need to creatively design the platform to minimize its impact on pathfinding choices, however).
The render distance of 10 equates to the distance limit for mob spawning, which basically means there's almost no room for mobs to spawn in lazy chunks (mobs in lazy chunks do not process AI and therefore will never despawn even though they still count towards the mob cap.) Setting it less means you get more lazy chunks and therefore more mobs in lazy chunks that cannot despawn, eventually enough mobs build up that will be there forever that more simply can't spawn.
second: i built on y=200 cuz, if u build farm in the surface u must light up area in 128 block radius to decrease to minimum chance mobs to spawn,
Do it the other way. Build as low as possible and then raise a ladder or waterfall to your AFK spot above the grinder. Mob spawning happens around the player, not around a specific place in the world (ie, your grinder), so you can basically have the best of both worlds by keeping yourself parked high enough to avoid having to light up the dark spots and keeping your grinder as low as possible to take advantage of faster spawn rates. Having the grinder lower also happens to mean that all the drops you're storing are more conveniently located for you when you need to use them...unless you've happened to build a skybase.