Hi there - welcome back and congrats on the birth of your new LO
Did you have any experience of reflux with your other LOs? It can take longer to sleep train when a LO has reflux but it is possible, you have to be really patient though because they get so uncomfortable when they are put down.
With the crib, I see you've elevated it but it takes really quite a steep incline to help with reflux, you might want to consider using both cot blocks (under the legs of the cot) plus a wedge (under the sheet) if you don't already use both. With the incline steep he might be more comfortable, you might also need a rolled towel under his legs to prevent him slipping down.
You might have more success if you shush/pat for an additional 20 mins after putting down, until he moves into the deep sleep part of his nap, this is advised n the early days of sleep training for all LOs but it might be that you need to pay even more attention to that with a refluxer. You might not have time for this for every nap (or any!) with 2 other LOs to care for so it may be more convenient for you and more comfortable for him if you do sling naps for now and tackle sleep training when he's a bit older, or perhaps one nap per day to get a habit started.
At this stage, he is so young, his meds might not be controlling the reflux so well, it could be he needs different meds or a different dose, or perhaps you can try eliminating things from your diet which might be flaring up the reflux when he feeds?
In terms of BF. If you'd like better advise than I can offer (I have so little experience) please feel free to post to the BF board. You can produce enough milk for his needs and there are tips for increasing and maintaining supply if needed. I wouldn't resort to top up feeds with formula (if that was on your mind) unless it proves absolutely necessary and after due consideration as this could really impact on your supply. I would post on the BF board for support before you make a decision. Here's the board:
https://babywhispererforums.com/index.php?board=12.0Hope this helps