Reducing leakage out the studio room door

There are kits. That particular one doesn't look to be too effective IMO. The last door I had done cost me about $1500-2000 seven years ago to have fit and put in.

#1 - start with a solid core door and trim it to be very close to the floor - less than 1/4". Preferably an exterior door (thicker).  And DEFINITELY solid core.  This will probably require re-woring the jamb, which brings us to :

#2. - even if its pretty square, pull off the door jam anyway and make sure that the cavities surrounding the door are filled with insulation / sound deadening materials. (this alone can get into a lengthy discussion).  This is also very important to the success of your project.  The size and square of the door and jamb and critical to getting a good seal.  The door has to be a super tight fit, and everything has to be perfectly parallel to make sure it is sealing all around.  A good door hanging guy with all the right tools and knowledge should be employed for maximum efficiency.  Making sure he/she understands the critical aspect of "tight" as it pertains to YOUR audio/acoustic standards - not general door hanging standards  - or you might as well leave the door open.

#3 get a metal floor threshold and complete surrounding metal doorstop that is "adjustable" with similar but denser sealing foam to the above. The one I have has VERY dense foam.  It's not the fluffy weatherstripping type of foam that you showed above.  I don't know who makes these doorstop / thresholds, but you mount them with acoustic caulking behind / under them, then they have adjustment screws that allow the entire doorstop that actually contacts the door to push closer / further away from the actual door itself.  And that's the secret key to success.  That and a good close fit and solid core door and dealing with the integrity of the wall / jamb surrounding the door.  There is also an acoustic membrane between the wood and the doorstop that fills in any irregularities in the wood jamb.

#4. Then you mount and adjust the doorstop / threshold until the door is a tiny bit difficult to "close" - you can't just swing it and have it close. You have to make a concerted firm push to close it. Over time, as the foam may compress, you can adjust the doorstop "closer" to the door to insure the seal. Having everything squared up and parallel and well built is key. Sound will go right around the door otherwise and you'll be wasting your efforts and money.

Beyond getting SUPER anal about it, this is the best thing I have found. It pulled SPL down significantly. Enough to make it worth the cost for me.

I’m going to add a bit to what Bill said, but first conflict alert, over the years a bunch of family members have been involved with Marshfield doors. Trying to soundproof anything other than a solid core door is both a waste of time and money. Generic kits are hit and miss at best, the best solution is a kit designed for a specific door. In my last building on of my neighbors was a Tony award winning composer, and a few of their neighbors did not nether working from home, building management had tried a generic kit and it knock the leakage a dB or 2. Since my friend the building manager new my background I was asked if I could help.
I had noticed the tag on the beautiful 10 ft solid core door indicated it was built by Marshfield, so I took a photo of the tag on the offending door and texted my cousin to see if they offered a kit for this specific model, they did. I was not involved in the purchase or installation, but we did a controlled experiment with a single quested at mesured level at 1m a calibrated C weighted SPL meter at 2 meters from the door with the generic kit, no kit and the Marshfield kit, generic -2dB Marshfield -10dB.
The generic did nothing at 35HZ the Marshfield -5dB. Yeah I’m a geek who else other than the Guys at GIK or Ethan would measure a fing door.

Yeah in the case of the apartment I think poor construction was just as big of an obstacle, on the other side of my unit the contractor had filled a 1/2in gap between a pillar and the drywall with silicone caulk, acoustical rope and Caulk solved that one.
You always have to rember air like water takes the path of least resistance.

On the acoustic curtain front, it's also a matter of any incremental improvement. I actually like that the lows go straight through (sorry neighbors!). it's part of the reason my room measures well I think.

But I'm kind of into the idea of getting acoustic curtains to open and close depending on my needs. I have fairly heavy curtains as it is along with a standard light blocking curtain liner. You think an off-the-shelf "acoustic" curtain would make a difference? Looks like these things are stuffed with recycled fibers to insulate them a bit.

Definitely thicker and heavier than what I have.

Try this dumb thing first:
www.peachloft.com/products/noise-pest-isolation-waterproof-door-bottom-seal-strip?_pos=1&_sid=da76d0992&_ss=r

And pair it with some self adhesive door seal thing.

Any gap around your door is going to let a lot of sound through. Yea, in a perfect world you could do all sorts of shit, but I’m guessing if you have house/roommates, this is prob gonna be more in your budget and the kind of investment you don’t have to sweat.

I have a serious room built in the basement as my tracking room but you wouldn’t believe how much sound leaked out of the bottom of the door - super old and not so level concrete floor.