I think this is overall a good interchange design. I like the creative way to separate traffic headed for South Tacoma Way from the I-5/SR 512 interchange. I also appreciate you squeezing in some direct connectors for future HOV lanes, though since your drawing is not to scale, I'm not sure whether they'd actually fit as you've drawn them. I'm also a little worried that the ramp to the P&R being so close would cause a weaving issue.
I do have one concern in that traffic currently backs up on the WB 512 to SB I-5 Ramp due to the tight curve on that loop ramp, which largely remains unchanged in this design. The HOV direct connector would help mitigate this by effectively adding another lane, but still being a loop ramp means the movement will have limited capacity regardless, due to the restricted speed around the tight curve. I see one reason you have for this design is to keep the clearance low for the McChord Field approach, however the interchange is about 4000 feet beyond the start of the approach surface for the runway, which gives a clearance of about 80 feet above the end of the runway at the surface's slope of 50 to 1, which should be more than enough to build a flyover bridge. There are certainly other issues that might make a flyover hard to fit in within the constraints of the interchange, especially without needing to take additional property, so the loop ramp option that you've drawn up still may be the best option.