Mine seems like the toughest plant. I have had mine maybe 20 years, it's quite large, and I don't even know if there's much soil left in the pot. It goes outside in filtered light when it warms up, gets watered sometimes, and then I bring it in in the winter. It usually dries out in the winter and the leaves might turn yellow and die. This winter it must be getting enough light and water from my cat's partly empty bowls that I toss on the plant. It is still green in a dim bathroom. My point is, once your plant gets established you might not have to fuss over it.

I'm not sure how to advise you to get it to survive at this stage. It does look like kind of a fine, dark soil. I do not know what the 5 1 1 soil is but assume it drains better. I voted to pot up in your new soil mix now but have no idea how fast it'll dry out and that's a concern since you're going on a trip.

I think the original potting soil I used was MIracle Grow.

Can I plant my Monstera and philodendron in cacti & succulent soil. This type of soil gives good aeration. Any advice much appreciated.

Trudy, products labeled as "cacti & succulent" mixes vary greatly from one another. Most or all of the major brands, however, probably claim to have good drainage and aeration but don't actually come anywhere close to living up to that claim. From personal experience, I can tell you that the Miracle Grow succulent mix that I purchased before I knew any better is worthless water-logged gumbo. It seems like it is just your average MG potting mix with a negligible amount of sand added. My Monstera is loving the 5-1-1 mix. Based on what I've read here and my experience, that is what I would recommend.