Honestly, it absolutely does not matter what order you introduce foods in. If you look at guidelines from different countries they are all massively different which says to me that none of it is evidence based and it's mostly bollocks.
I had my first 10 years ago in the UK and my second is just one, born in Germany so I have looked into this recently.
All of the guides from baby food companies make it sound overly complicated so that you will buy their products which "make it easy". It is fine to just do it your own way.
Grains are very difficult for a baby to process and digest, generally they will pass through untouched. So I would not give too many grains, they can be used as a "vehicle" for more nutritious foods, such as toast with avocado or cream cheese spread on, or pasta with sauce on it. Baby rice and/or porridge are really just a way to give texture to milk/purees, and may be fortified with vitamins.
Nothing bad is going to happen if you give your baby foods in the order not recommended on the internet or by the weaning book. Don't stress, make it fun.
The UK's guidance on what to introduce (or not) before 6 months seems to be based on fears of allergies - but more recent research suggests that it is beneficial in terms of preventing allergies to offer allergenic foods before 6 months. Other countries recommend that meat, fish, dairy and so on are introduced before 6 months with no widespread difference in allergy rates.
The idea of introducing foods one at a time is so that you can identify if anything causes a reaction, but this isn't necessary unless there is a history of allergies in the family. You can do it if you want to though.
Watch salt levels - strictly no more than 1g per day up to 6 months, 6-12 months on average up to 1g per day, 1-3 years on average up to 2g per day. If a label only shows sodium, multiply by 2.5 to get salt equivalent. Be aware that UK children's foods tend to be low in salt as baby food manufacturers were called out on salt levels a couple of years ago, but they still cram masses of sugar into their foods. Anything with added fruit juice concentrate is going to be high in refined sugar, which is bad for developing teeth. Petits Filous are a bit out of fashion now due to the sugar - I have given them to both of my babies but you're encouraged to use plain greek yoghurt (with a bit of fruit puree mixed in if you like).
You can give finger foods before 6 months as long as you follow BLW guidance - sitting up rather than reclined, allowing them to do it completely by themselves (if they can't, they're not ready) and always with supervision.
This is pretty nice for mashing up portions of your own food if you're eating something suitable. But be careful with buying from amazon as there are a lot of Chinese products on there which probably won't be BPA free, even if they say they are.
WhyBird It's a little different in the case of reflux, with reflux the milk isn't managing to nourish the child as too much of it is being brought back up. So the idea is that solid foods (purees) stay in the stomach longer/are less likely to be brought up as they are thicker, so the calorie density isn't really the problem.