FTM looking for advice, tips and tricks on babies first foods/solids, Spit up, and hand sucking!!

Some info on My daughter, she is 16 weeks old and just drank 8 ounces for the first time this morning. Right now she eats 30-36 ounces a day on demand. She sleeps about 10 hrs a night without waking up. She is a supported sitter, she holds her self up very well and head no problem. About 3 weeks ago I started to give her about one tablespoon oatmeal cereal in her night time bottle to keep her full longer. When I try to give her some in a bowl, spoon fed, she doesn’t enjoy the taste but will slowly eat it. She will do the tongue thing where she pushes it out of her mouth if needed. Her weight a week ago was 13.7lbs. She has been on Similac Total Comfort for over 2 months but the past few weeks she had started to spit up a lot more so i slowly introduced Similac Spit up. At first I thought it was like night and day, then it went back to before. I have started to do 4 oz Similac Spit up and 2 oz total comfort - told to try by a nutritionist because she had been getting fussier than normal since new formula; which I have a seen a little difference but she still spits up a good amount here and there. She also just started to eat and suck on her hands a few weeks ago. Now it has drastically changed to doing it as early as 20 mins after eating. She doesn’t suck/eat anything else and doesn’t like pacifiers. 75% of the time her burps are good, but sometimes there’s none. Her spit up can come immediately after or up to 45 mins after. AND I sit her up for min 25 mins after eating.

Any advice about the spit up? Any moms here mix 2 types of formula and why? When should I start introducing stage one baby foods? How many times a day should I give solids and how much? How long or how many feedings should I stick to one specific kind of food? Fruits or vegetables first? Any wedsites books or meal plans to follow/reference? I just want to ensure she is eating and keeping down enough to maintain weight and nutrition! Please no hate. TYIA😊