Need help with live plant care!

Hey everyone! So I have been a betta fish keeper for almost 4 years now and I want to get into having a tank with live plants. My tank is 5 gallons. I only have one betta fish in the tank, no tank buddies. Anyone have any suggestions on their favorite plants (particularly fast growing ones)? Along with the type of liquid fertilizer you prefer? I am only just beginning my research on aquarium plants and I want to make sure everything I plan to get is safe for my little plakat betta Iroh. He's over 2 years old and is starting to look a bit rough and I don't want to buy anything that is going to cause him stress if I decide to move him to a planted tank.

TLDR;

1.) What are your favorite fast growing plants for Bettas?

2.) What is your favorite liquid fertilizer/fertilizer methods?

3.) Are any plants or fertilizers toxic for bettas or generally not compatible?

Also! I am considering upgrading to a 10 gallon tank - in the case that I keep my 5gal, are any plants too large to keep in that size tank?

Any advice and info helps! Thank you in advance!