This is the main one I watched. It's for guitar and it includes hammer ons and pull offs.
The same principles apply for the uke but I alternate the fingers and walk them around the strings for the scales as well as using the above exercise. I saw this in another video for ukulele techniques but I can't find it. Ukulelezen is a good youtube channel and so is Ukelikethepros and Tenthembspro for vids on learning new techniques and songs. Ukulelezen sounds like he's talking to a foreign call centre sometimes, but they can all play and have heaps of content for free. I haven't even scratched the surface really.
I wouldn't anchor the thumb on the uke. You will need it sometimes. If you practice with the thumb on the uke it will be too hard to use when you need it. The strumming hand should hover over the strings with no contact on the uke. If you need to stabilise your hand squeeze the uke between your body and your forearm where ever it is comfortable. Or you can consider getting a strap if it really bothers you. If you absolutely have to touch the instrument with your strumming hand I'd use the pinky.
Of course for every rule of thumb (I'm Punny), I bet you can find a pro that breaks the rule but I recon you'd go a long way to find one who is predominantly using the thumb for anything. Don't get me wrong, you will still need it and theres plenty of times where its good to use but it's not your go to. I still have quite a few songs where a assign a finger to a string, I probably didn't explain this well in the previous comment but it looks like the below -
G string - Thumb
C string - Pointer
E string - Middle finger
A string - Ring Finger
Each finger plucks the assigned string only. (ie - Aint No Sunshine When She's Gone, House of the Rising Sun)
When a song needs to be fast though, alternate picking with the pointer and middle finger has the highest ceiling for speed, particularly if there are a lot of notes played consecutively on the same string (ie - Fur Elise, Under the Bridge, Cotton Eye Joe. I'd be stunned if you could reach top speed on these three songs using any other method but my wife has an extensive list hidden away of all the times I've been wrong so I don't know why I am still so sure of myself.)
Also like I said previously, I use the thumb and pointer finger for pinching double stops, you will need this on a lot of songs where for example you need to play notes plucking only the C and A strings (Think - Sloop John B, Waltzing Matilda)
To be honest, I'm impressed you got to level 5 using your thumb as default, you have probably practiced with it more than me and may be far better with it than I can understand. My advice is based on what has worked for me so far and may not be right for you. I'm hoping someone with more experience chimes in and offers alternatives or supports what I've said.
One last thing I've seen but it doesn't work well for me, but it could work for you -
G string - Thumb
C string - Thumb
E string - Pointer
A string - Pointer or Middle finger
I find my thumb too slow for this to work well but it might be just right for you.
Let me know what works if anything of the above, I'd love to know if what I said worked for someone else and also what didn't really fit.