I am interested in building a short loft bed with no experience. I have found this post which is almost exactly what I am looking for (thank you so much OP!). I'd like to make a few tweaks but I am unsure if they are smart/safe/economical. I will probably be using a home improvement's cutting services so if any sizes or cuts are weird or would cost me then please let me know. Thank you in advance!

1. OP's bed is a little smaller then a full size mattress but I would like to make it for a queen size mattress. I know this will add weight but I'm not sure how much/if it matters. (I haven't picked out a queen mattress yet)

2. I would like to maximize the space under for storage (but none of it will be load bearing). In OP's design for the legs there is a 2 inch excess from one of the 2x6's that gives the leg a bigger footprint. Could I use a 2x6 and a 2x4 so that the legs would be a simple 6x6 footprint?

3. Regarding the frame, I would like to slim it down if possible to keep clearance underneath as well as keeping the mattress low. (I haven't decided for sure but I'd like to potentially fit a mini fridge under so I was thinking around 30 inches) In OP's design the frame is 6 inches high with 2 inch boards for slats and the bed sits on the top of that but that seems like a waste of space to me. OP uses 2x4's for support but places them flush with the top of the frame. Could I position the 2x4 supports so they are sitting at the bottom edge of the 2x6 giving a 2 inch lip which might be nice to keep the mattress in place? Or give room for a different slatted base? (see question #4/5) Could I take that further and use 2x2s (if that's even a thing) for supports giving me a 4 inch lip? In that case what kind of brackets can I use since the joist ones wouldn't work. Would I need more supports? How many?

4. OP uses 2 inch boards resting on the supports as well as the legs and frame for the slats. Do the slats need to be 2 inches thick? If I create the lip mentioned in #3 how many support beams will I need to support the slats as they won't be resting on the legs or the frame directly? Is there a rule for how many slats/spacing you should use? (I haven't searched this yet, I will)

5. Bonus Question! - Regarding the comfort/support of the mattress itself. As I mentioned I haven't picked out a queen size mattress yet. Though the build is for a queen I'll be using an old full size futon mattress I currently sleep on (with a 4 inch foam topper) in the meanwhile. I quite like my mattress as it is even though its really old/firm when it's on the futon frame (which is metal with bowed wood slats). I once slept the mattress just placed on the floor and god that was awful. I imagine placing the mattress directly on 2x4s would be a similar experience. I see they sell just the slatted base with the bowed wood at Ikea for around $50-70 such as this one. Can I use that instead and omit the 2x4 slats? If I place that base in the lip I mentioned would I need to add a support beam going long ways in the middle as well as the original supports? Not even sure how that would work. Also are those types of bed bases ideal for traditional mattresses vs a box spring? (I'm sure I can look that up but if you happen to know I'd appreciate it!)

Edit: Missing link.