I'm wondering what the max bioload a tank can support under a fishless condition
That would depend on the total surface area within the aquarium, substrate, rocks, decorations, and, most importantly, filter media (specifically biological filtration media) available for the beneficial bacteria to colonize.
...if I'm using a fishless cycle and the ammonia is kept at 8ppm until the same dosage now has 0ppm of ammonia but then add even more ammonia so that the ppm rises to let's say 8ppm, you're not recycling the tank, you're just adding more of a bioload to handle more fish, essential, right?
If your aquarium is able to process 8ppm of ammonia (and subsequent nitrite) within a 24hr period or less, I would say it is ready to support fish (which is different from having it start at 8ppm, and then eventually attain 0ppm after several days). Adding a particular quantity of ammonia in one single dose to bring ammonia BACK to 8ppm is something that won't typically happen in a aquarium containing fish. But if the aquarium is still able to process that accumulation of ammonia within a 24hr period (again), it may be considered fully cycled.
When preparing to stock a lot of fish all at once, I find that building up the beneficial bacteria colony helps immensely. If my 125gal fishless cycling tank can process 12ppm ammonia every day consistently for a week or more, has no nitrite, and has accumulating nitrates, it's ready for fish (as in a dozen or more 3"-4" inch fish added all at once).
I've tried it and done it on many occasions. My goals were to create more ammonia production than the eventual fish would produce (i.e., a tank that can process 12ppm of ammonia/nitrite daily can support a lot of fish added all at once; even the fish in my most heavily stocked tanks don't produce that much ammonia).
A bioload that is too large will cause nitrates to rise fast, which is unhealthy over the long term.
That's where water changes come in. Nitrate production is an expected consequence.
For example, if one desires to add a half dozen 11-inch fish in a 180gal tank (say, some good sized african cichlids like full grown nimbochromis venustus). Based on the behavior and requirement of that species, that tank is considered adequate and ideal for them, and isn't even ovestocked.
In an established tank they'd probably produce 50ppm of nitrate a week (or maybe less). If the ratio of ammonia:nitrite:nitrate are the same then at roughly means they're producing 7ppm ammonia daily. If during the fishless cycling process the tank is conditioned to process 12ppm of ammonia/nitrite within a day, that tank for all intents an purposes would be ready to support a half dozen 11-inch n. venustus added all at once. If it takes days for 12ppm to be processed, the tank wouldn't be ready for that amount of fish introduction.
Keep in mind that building the bacteria colony to the point where it can process this amount of ammonia and nitrite will take weeks (8-12+).