So i have been struggling HARD with the same thing, i finally got things pretty dialed with these settings and the rivergen mod, I'll include the rivergen JSON as well cause that took alot of tweaking. Top code is the worldconfig settings for server, second is the json file (just delete the data rivergen makes and replace with that). Hope this helps!
https://mods.vintagestory.at/rivergen
"landcover": 0.3,
"oceanscale": 3,
"upheavelCommonness": 0.4,
"geologicActivity": 0.1,
"landformScale": 1.2,
{
"disableFlow": false,
"minForkAngle": 10,
"forkVariation": 35,
"normalAngle": 20,
"valleyStrengthMax": 1.0,
"valleyStrengthMin": 0.8,
"noiseExpansion": 1.5,
"riverPaddingBlocks": 128,
"landScaleMultiplier": 1.0,
"minSize": 10.0,
"maxSize": 50.0,
"minNodes": 8,
"maxNodes": 20,
"riverGrowth": 3.0,
"downhillError": 1,
"minLength": 150,
"lengthVariation": 200,
"zoneSize": 256,
"zonesInRegion": 128,
"riverSpawnChance": 75,
"riverSplitChance": 35,
"lakeChance": 30,
"segmentsInRiver": 3,
"segmentOffset": 40.0,
"baseDepth": 0.14,
"riverDepth": 0.04,
"heightBoost": 8,
"topFactor": 2.0,
"riverOctaves": 2,
"riverFrequency": 0.0075,
"riverLacunarity": 3.0,
"riverGain": 0.3,
"riverDistortionStrength": 10,
"riverSpeed": 4.0,
"maxValleyWidth": 75.0,
"oceanThreshold": 30.0,
"fixGravityBlocks": true,
"boulders": true,
"riverDeposits": true,
"riverDepositsBlocks": [
"game:rawclay-red-none",
"game:rawclay-blue-none"
],
"clayDepositFrequency": 0.2,
"gravelBeaches": true,
"valleysV2": true,
"ignoreStoryStructures": false,
"cacheZones": true,
"RegionSize": 32768,
"ChunksInRegion": 1024,
"ChunksInZone": 8
}