Personally I'd keep away from all grains to start with
This. I agree with what Sara said about starting with veggies too.
I started solids a bit early, 5.5 months but if I had that time again I would probably start sooner. Not MPI, just reflux, but he only ever took half the guidance amount of milk no matter what. Introducing solids was extra food for him (milk didn't drop, if anything it went up some because he was more comfortable) plus he was interested, took to it with gusto and did so much better with solids than without.
Rice was a big no no here though. I know a lot of people start with baby rice but it made DS puke, gave him discomfort and really didn't work well - that was every type of rice I tried. The powdered sort of baby rice, rice pudding (later, made with his formula, he seemed to like the first bit but it didn't work out), rice crackers made him puke, and now he cannot eat rice (he will try a mouthful say for chinese new year when they serve up various bits at nursery and he did not puke but he clearly does not like it or do well with it, it's clearly not just a fussiness yk?).
The big indicator for me really was that DS was screaming at me from 4.5 months every time he saw me with food. He wanted it. The month I waited was hellish with me hiding to eat, often just not eating and him screaming blue murder every time he saw food. (I only ever tried to eat in front of him immediately after he'd had his fill of milk, he was supposed to be full, he'd start screaming, I'd stop eating and re-offer milk, he'd refuse the milk and continue screaming...until I gave up on my meal and put the food out of sight...urgh).
At the time I felt the 6 month guidance was a strict rule which I didn't want to break, but now I feel more like the guidance of 4-6 months is more realistic and LOs are ready in their own time. I agree with the general guidance but also more able to see babies are all ready at different times and respond differently too.
Don't know if this helps at all, just sharing my experience.
ETA DS also had to eat every 3hrs when on just milk. He could no way wait another 5 mins. We didn't hit 4 hr milk until solids were established.