On your specific build, I'd say skip the "apartment" stackables and get the full-size stackables. The full-size ones are just so much better made that for the few inches you save, you'll be better off in the long term. That end wall where the picture is
should either be storage or glass. It looks like you have space to do floor-to-ceiling storage there. Storage is going to be your key to living in a tiny space without going insane. Without you saying, it looks like you'll be on-grid but I'd suggest preparing for power outages if you're in a rural or remote area. That means some oil lamps or candles, and alternative heat sources like a propane heater. The heat might be uncomfortable but there are ways to work around it and it won't bust your pipes. We came through Snowmageddon in a 2 season RV that never froze despite the outside temps hitting -9F for a few days because we were prepared.
I also recommend a shed or other outdoor area to store things that don't necessarily need to be temperature controlled - bikes, sports equipment, out-of-season clothes, and out-of-season bedding. Space bags and plastic bins are your friends here. Toss in a
dryer sheet, vacuum them up, bin them up and they'll be ready when the seasons change.
Not so much a specific suggestion but a tiny house living in general. Try to leave yourself space to build on or attach another container, etc. so that you don't have to start from ground 0 if your needs change. That might mean doing something a bit different with the roofline to accommodate a potential future build or installing storage that you can relocate if you add on to the existing building. That said, glass is your friend. Yes, drywall is cheaper but being able to see out makes things feel bigger. Having a good outdoor space that's at least 2-3 seasons if not 4 (depending on your location) is also quite helpful. In all but the coldest part of winter, I prefer to cook outside whenever I can.
A single person in 380 square feet is doable if you're very disciplined about making your bed, folding it back up, putting everything away as soon as you're done with it, and generally being extremely tidy. There is a rigor to living in that small area that few people talk about.
You also need to be extremely disciplined with your possessions. Everything needs to have a reason and a place or it should be jettisoned. If something new comes in, it's because something old is going out. You'll want to squeeze in storage space where ever you can. You'll also want to be sure that you can do all the basics inside (cooking, laundry, eating, etc.) so that when the weather's bad, you don't have get out into it for basic needs.
Add another adult person and things will get crowded quickly since the space is already filled with your items. That's going to mean reaching an agreement with who's going to get rid of what items in order to merge households. Bring in a baby - with the crib, playpen, diapers, etc., or a toddler with their toys and now you don't fit in 380 sq feet so well. There are two things that ruin most couples faster than anything I know of - fights about money and the home being a constant mess.