#1
Hi everyone! I recently switched to finger picking and I find it funner.

BUT my skin really soft and I keep getting blisters on my right hand thumb, even after 5 minutes of playing!

How do I get a callus on my strumming hand thumb?

My fretting hand don't hurt

I fin long nails hard to maintain and gross, so thats not an option
#2
I assume that you have an acoustic guitar with metal strings. I have started fingerpicking on classical with nylon strings which are far more gentle to your fingers. Put them on your acoustic and after a couple of months you will be more than ready to switch back to metal ones.

But if that doesn't work for you, you could try playing with a thumbpick like many famous fingerpickers did. It also adds great volume to your bass.

What kind of fingerstyle do you play? When I started fingerpicking I had more trouble with my fingers than my thumb.
#3
The thumbpick is the natural route; as shatterhand says, many famous fingerstylists use them.

Or... I don't have any right-hand callouses at all. I use the same technique classical players do; a combination of flesh and nail that produces a clear tone. Just enough nail on the thumb... No "callous" needed.
But many find that tedious, and use the bare fingers.... Are you using the thumb to actually "pick" individual notes, or as more of a strum... A thumb-brush to play chords?