You don't need to split the raster at all. There are a couple ways to do it. One is to use two tools in succession:
Use Reclassify to convert the elevation values in your raster. Use the natural breaks you've already found as the bins to reclassify your raster to just 3 values: 1 (lowland), 2 (middle) and 3 (upland).
Use Raster to Polygon to convert your ...
As noted in comments, you likely need to build overviews.
Overviews and inner tiling optimize two different types of access:
Having overviews optimizes the most the case when you're trying to look at the the whole raster on a small map, that is, the case where the GIS software needs to produce an output image that's smaller than the original raster. ...
Create raster with cell values of 1 using raster calculator, e.g.
Draw section at stream 'mouth' and use above raster as cost surface and section line as input feature for Cost Distance tool. You'll get something like this:
Convert raster to points and remove unwanted points downstream from section line. Sort points in ...
You can use Split Raster in the Data Management toolbox. https://pro.arcgis.com/en/pro-app/tool-reference/data-management/split-raster.htm
Within the tool you can specify the output raster size, number of tiles, or use a polygon feature class (i.e. a fishnet). The tool requires that you specify an output folder and base name for each resulting raster.
I am unsure how accurate you want your results to be, my understanding is that you can do one of two approaches:
Find known water level points for the rainfall event and create an interpolated surface, do a quick raster calc to find areas where terrain is higher than the water surface and youll get an approximate extent of flooding.
The more standard ...
You can use the Extract by Attributes tool in the Spatial Analyst toolbox to create a new raster from a range of data. You will need to sort out ModelBuilder iterators, or loop the Extract by Attributes tool with Python to get each of your classifications into different files.
You can also use BlenderGIS plugin to import georeferenced images in Blender. Also, BlenderGIS can download SRTM directly into blender from inside Blender if you want. It is a very useful plugin dedicated to integrate GIS data with 3D visualization.
Here is a blog: Shaded relief with BlenderGIS (2020) that explains on how to use BlenderGIS step by step to ...
With pretrained weights, such as those used in the example, you cannot just add another band / layer of information. This is because the pretrained weights are made for standard RGB imagery and does not work with four layers of input.
As such, you have two options:
Drop the pretrained weights
Reduce your dimensions from four to three. Options are dropping a ...