Team, it's been a hot second and a dead thread for the last few weeks (months). Bed is complete, new mattress and all. Pretty happy with end result, and really appreciate the input you all had. Wood is a mix of white and red oak for exterior, with pine for center support and slats. Finished with linseed oil. Photos below:
Finished, installed in it's natural habitat.
Close up of finished corner joint. Small gaps there, but it's a solid joint with the metal brackets installed on the inside. Bed breaks down into 5 pieces: left and right runners with legs, center support runner, and head/footboard runners.
Corner glue-up. Tie-down straps and microfiber cloth. Able to use the strap to adjust pressure location on the legs to keep them perpendicular to the frame.
Center support leg. Played around with a few ideas of how to join this, but went with hogging out center material with drill press (which is a blast) and then finishing with a hand saw and a dull harbor freight chisel. I need to invest in a sharpening stone. Lot of tear-out internal, but guess who sees that? Not you nor I, with the bed all together. Our little secret: HF tools + inexperienced hobbiest don't equal perfection. The 4x dowels (with the drill press again) lock this leg into place, even with the gap/slop there. No wiggle. No shifting.
When I started this project, someone said "don't overengineer it". I don't know if this qualifies as that or not, but I definitely didn't want to worry about this joint loosening over time. Really happy with this corner bracket from Rockler. Locks the joint into place, easy enough to disassemble, and hidden from sight.
Thanks for stopping by, and thanks again for the help along the way. Sorry it took so long to report back! Shifted gears starting training and had to wait until holiday break to get the time and headspace to finish this.