Applying textures to my custom terrain

Hello! I've coded my first procedural terrain made with noise, some multithreading, random seeds and a subdivision of the map in "chunks", aka custom meshes. I followed most of the tutorials by Sebastian Lague on Youtube, but I don't really like the way he handles the "painting" of the terrain.
My question is: What is a nice texture system for my terrain (aka custom meshes) where I can modify every texture by the player at runtime (digging, planting grass etc) and choosing every tile by code (not just by height but by biome, position etc)? I also need to save this data and load it back.

Spoiler: Additional information (and what I'm currently trying):

I currently create a texture that just choose his basic color by the height of the tile. Most of the tutorials online shows how to use a shader to do this and paint a texture instead of a color in the terrain, but all the shaders do this automatically by height, and I want to be able to do biomes, different types of grass/textures for a zone, modify the ground's texture with the player etc.
However it seems like I'm very far from the solution, because I have no clue on how to do this. I have no idea on how to use shaders to do a dynamic and player-influenced terrain texture, so I tried using Texture2D. The main problem is that even small textures with low resolution are really time expensive. Every chunk has its own texture, creating it costs a lot of time (and with a very low resolution! like, 10x10 pixels per square meter), and lower the fps by a lot. And I don't know how to scale down my external texture (e.g. the grass texture), because they are pretty big (images 3000x3000) and I am trying to cut off a square of about 300 square pixels (with "GetPixels) and then applying that to a small section of 10 squared pixels (the tile in the chunk's texture).
So what I get is something like this: every tiles is scaled really bad into something that recalls the original image, but it sucks.

(The cube is the average dimension of a player: that shows how big are the pixels in the terrain).
And the effect that I'd like to get is:

(That's my grass texture.)
I don't even know how to "melt" two different textures together, to make things smoother in the biome's borders (e.g. grass and sand).
Tl:dr I'm doing in a wrong way something that is slow and is just a small part of the big problem: i'm completely out of the right path.
If someone can point me to the right direction I would really appreciate that. I've looked online for tutorials but everyone uses just some assets, or some shader, in a way that's not doable with what I'm doing.
The basic effect that I'd like to get is something like Valheilm's terrain: a nice and procedural terrain with a nice texture.

Spoiler: Other images:

This is my current terrain without any experimental texture. Just tiles with a clean color: green is grass, yellow sand, brwon mountain and blue the sea. Every tile is a pixel in the Texture2D of the chunk (about a 95x95 texture, per chunk. With my texture is a 400x400... per chunk.).

Trying my system with the grass:
(that's basically a zoom-out of the previous image).

It looks kinda okay when you look it from far away, but it doesn't look ok when you're close. And it lags a lot.

Thanks a lot for every help you can give me! If you can point me to the right direction with tutorials, tips or anything I would really appreciate it.

 

i tried an idea with texture arrays

Choose
texture_A from vertex.Red
texture_B from vertex.Green
then I just grabbed UV(red) to test the LERP on the choices with a procedural 'quad' (line with 2 segments)




aside from the LERP being a simplistic blend artistically it sorta works in theory

this procedural geos batch (at least as far as I can tell technically) and use 2 texture fetches so should be 'cheap' in terms of pixelA x pixelB in the frag.. i think.

depending on how you generate or import terrain, you may need a 3rd image that is made at runtime, the Red and Green values become your array index lookup.. as well as computing the borders. (youd really only want to do a LERP on borders and use a simple array material for the pure zones)

this also get's more complex with corners that have more than 2 textures to blend.
but this is getting outside of scope for me, i just had an idea about TextureArrays I wanted to test with

 

This seems a nice solution to blend two biomes together but.. I still need a system for my world. Because my texture system is pretty broken (and laggy)...