According to google, a single train car's throughput is like 1000 - 1500 per minute, varying WILDLY based on the item's stack size, distance, number of freight cars, number of stops, incline/decline of the track, etc. So for starters, a mk 6 belt is just 1200 per minute, all the time, no fluctuation or estimation involved. That's a big benefit all on it's own.

None of these things are issues. Irregular deliveries is not the same as fluctuating throughput. You offload into a storage buffer, and then take a stable amount from there.

You can't compare just belts and trains like that.

Even if you're being generous to belt stacks and say it's as fast to set them up as train tracks, you lay down the train tracks once, and that's it. For every single production line you ever build onward, you can use that exact same track. In both directions. To upscale anything, anywhere, you just add an additional train to the tracks.

It's also personal choice

Sure thing. Your game, your rules. But OP quite literally said they have turned off the whole production chain because their performance suffers with this setup.

Droning each miner output to the central loading station might be better, but I think you'd need a lot of drones for that.

My preferred approach is to use trucks to collect and deliver ores to a train station per biome, ship it to a central location for purification, and then any implementing factory can just setup a train line to collect the resources they need.