GEE: How to classify Sentinel-2 RED band into 5 classes, with class boundaries being dynamically derived from respective MIN-MAX range - decision-tree

Question

I'm trying to develop a script that classifies the RED band for an AOI of a Sentinel-2 image into 5 equal classes, based on the AOI-specific spectral range of the RED band.
I managed to extract the MIN and MAX values forthe RED band of the user-defined AOI and, based on that, to define the respective class boundaries as variables. I also found several classifiers which can do the trick if the class boundaries are given as numbers, but they don't run if I give the variable for the class boundary, which would allow the classification to run automatically, once the user defines the AOI.
Is there a way to make, e.g. the ee.Algorithms.If function or the ee.Classifier.decisionTree to run with variables instead of hard-wired numbers?
Any suggestions will be appreciated.
Here the code, with a de-activated, fruitless attempt to the classification at the bottom.
// add Durban Image
var durbs = ee.Image("COPERNICUS/S2_SR/20211015T073901_20211015T080043_T36JUN");
// Draw polygon to define Area of Interest
var AOI = geometry3;
// Define the visual parameters
var visParams = {bands: ["B4","B3","B2"],
gamma: 1,
max: 1422,
min: -92,
opacity: 1};
// Set map centre
Map.centerObject(AOI, 8);
//Add images to check
Map.addLayer(durbs, visParams, 'Durban');
// Clipping of S2 RED band to AOI extent
var S2subset = durbs.clip(AOI).select('B4');
// Display clipped image in grey scale
var visParams2 = {bands: ["B4"],
gamma: 1,
max: 1422,
min: -92,
opacity: 1};
Map.addLayer(S2subset, visParams2, 'S2_B4_subset');
//extract MIN values
var min = S2subset.reduceRegion({
reducer: ee.Reducer.min(),
geometry: S2subset.geometry(),
scale: 10,
maxPixels: 1e10
});
print('band MIN', min);
print('MIN value in B4', min.get('B4'));
//extract MAX values
var max = S2subset.reduceRegion({
reducer: ee.Reducer.max(),
geometry: S2subset.geometry(),
scale: 10, // scale defines pixel size
maxPixels: 1e10
});
print('band MAX', max);
print('MAX value in B4', max.get('B4'));
// Result of reduceRegion is a dictionary.
// We can extract the values using .get() function
var max_B4 = max.get('B4'); // this is just a replication of line 48ff above
var min_B4 = min.get('B4'); // this is just a replication of lines 37ff above
// A dictionary of variables to use in expression.
var variables = {x: max_B4, y: min_B4};
// Create boundaries for 5 classes based on AOI-specific RED band range, derived from max_B4 and min_B4.
var range = ee.Number.expression('x - y', variables);
print('range', range);
var class_width = ee.Number.expression('(x - y)/5', variables);
print('class width', class_width);
var class1_min = ee.Number.expression('y', variables);
print('minimum', class1_min);
var class1_2break = ee.Number.expression('y+((x - y)/5)*1', variables);
print('1-2 break', class1_2break);
var class2_3break = ee.Number.expression('y+((x - y)/5)*2', variables);
print('2-3 break', class2_3break);
var class3_4break = ee.Number.expression('y+((x - y)/5)*3', variables);
print('3-4 break', class3_4break);
var class4_5break = ee.Number.expression('y+((x - y)/5)*4', variables);
print('4-5 break', class4_5break);
// SCRIPT RUNS FINE UNTIL HERE, BELOW ONE FRUITLESS ATTEMPT TO USE
// THE ABOVE BOUNDARIES FOR THE ACTUAL CLASSIFICATION: .... PLEASE HELP!
var class_1 = ee.Image(ee.Algorithms.If(S2subset<=class1_2break,
1, 0));
print('class1', class_1); // this was a test to see what happens
/*
var variables = {
a: class1_2break,
b: class2_3break,
c: class3_4break,
d: class4_5break
};
var DTstring = [
'1) root 9999 9999 9999',
'2) S2subset<={ee.Number.expression(a, variables)} 9999 9999 1 *',
'3) S2subset>class1_2break 9999 9999 9999', <----- I tried to use my variable from above here, unsuccessfully
'6) S2subset<=class2_3break 9999 9999 2 *',
'7) S2subset>0.4 9999 9999 9999', <---- the "0.4 etc. come from an example script
'14) S2subset<=0.6 9999 9999 3 *',
'15) S2subset>0.6 9999 9999 9999',
'30) S2subset<=0.8 9999 9999 4 *',
'31) S2subset>0.8 9999 9999 5 *'].join("\n");
var classifier = ee.Classifier.decisionTree(DTstring);
var reclassifiedImage = S2subset.select('nd').classify(classifier);
*/