In terms of the microorganisms that handle cycling and a substrate, here are a few facts to consider.
These organisms need oxygen among other things. Wile they will colonize a substrate, they will do so only in a maximum of the first inch and likely a bit less. So if you are not going to use plants which must be put into the substrate nor are you willing to grow them in pots, then you only need about 1 inch of sand or gravel for cycling considerations.
Next, the cycling microorganisms are somewhat photophobic. This means they dislike much light.So, in a completely bare tank you will mostly only have them in a filter. Unless you have a specific reason for not wanting substrate, I would add some. But if you need to be bare bottom, then add some decor such as wood or rock. This will give the bacteria and Archaea a place to colonize out ot the light. Yhe Archaea are even more photophobic than the bacteria.
The way you are set up now means that cycling your tank will take the longest amount of time. The microorganisms need to end up in the filter to begin multiplying as nowhere else in the tank is hospitable for them.
Pne more thing to consider here. By having a "blank tank" there is just the one location for cycling chores to happen, in the filter. Any[rob;lrem there omce the tank is stocked means big trouble. I would suggest of you intend to stay bare that you need more than one filter and that your choice of filtration be one where a lot of biomedia volume is possible.