Looking pretty good even if the screenshots are a little cherry picked
Some tips from a seasoned random mode masochist:
Volatility1 and Volatility2 are not considered in the SmoothRadius calculation, when two biomes with different vol1/vol2 settings collide you will see some ugliness, you can minimise this by having border biomes that have more 'neutral' vol1 and vol2 settings, so instead of one giant border of ugliness you have a more gradual return to 'normal'
Random mode can be a fickle beast and isle biomes and their sizes/rarities especially so. If you find yourself wanting to create a mountain range from isle biomes, and that 98/99 rarity for your mountain isle biome is too much (mountains 'blob' together too much) and 97 or below is too low (mountain range does not have many mountains), consider creating a duplicate of your isle biome and lowering the rarity of both. Since they will essentially 'fight' over the space in your 'parent' biome you will end up with different looking results to tweaking the rarity of a single biome.
It is possible to create a mountain range 'in reverse' where the 'parent' biome is high up and the isles within it create valleys instead of peaks. This can give a very different look and is worth experimenting with imo.
Surfacepatch as you say is quite limited. You have found one good alternate method for varying up the surface blocks (tiny bo3s), I find the Ore() resource to be invaluable for this, especially multiple uses of it with different size/frequency/height settings. I often use it to decorate the surface in Biome Bundle. Examples with varying settings:
You can also Plant() snow in colder biomes to great effect:
This one is a little more complicated, but you can create 'brush' bo3's with Sourceblock set to only affect certain blocks, I use this in caves quite a bit to 'paint' the interior. In this example it is used to create the 'striped' walls without having to replace blocks in the entire biome, the bo3 is basically a big funky stripey blob which places after the custom cave bo3's, it uses block detection to detect air but does not replace the air blocks:
Got a bit carried away there and I don't want to spam your thread with images, but I could go on.. and on... heh
Hope that was helpful, and good luck you're off to a great start.