I endorse Ametrine's strategies of drawing boundaries around certain portions of the day for DS to self-entertain, and turning over primary responsibility for your DS when DH gets home (give DH about half an hour to decompress from the workday, though).
In addition, I wanted to comment on this...:
And it does seem to me that he also has the traits of being introverted and having a high-reactive nervous system (things are often too loud, he watches from the edge of the playground before becoming involved, etc.) so I feel really confused about what's going on here and now I can help.
... because my DD is also one who watches from the edges before becoming involved, but being socially guarded should not, in her case, be confused with being introverted. She is wary of strangers, but once she makes up her mind about people, she's energized by being a part of a crowd, and wears the other people out. When she doesn't have a playmate she complains of being lonely.
So it's possible you're misreading these cues in your own DS as introverted behavior.
I also wouldn't lump in his sensory responses to sound as an extroverted/introverted behavior, because sensory sensitivity is not controlled by personality. Sometimes it's the other way around, though, where your DS may appear introverted because his senses are being overwhelmed, but placed in a quieter environment, he may respond quite differently.
Lastly, I would hypothesize that his sudden spurt of growing intensity in seeking your attention may be his response to some of your sense of being overwhelmed. He could be picking up on your reluctance to engage (you'll be sending him nonverbal and verbal cues on this, whether you know it or not), and that may be causing him some anxiety about being rejected. He responds by seeking
more of your attention for self-validation, which in turn causes you to feel
more overwhelmed, and the runaway feedback cycle continues.
The solution there is, as mentioned before, healthy boundaries. When DS understands that you're not rejecting him, you just need to get X done, or you need a little time to Y in order to be ready to spend quality time with him, and then that time you do spend with him is high quality time, then the psychological health of both of you is greatly enhanced.
On weekend mornings, I have a two cups of coffee rule. Anyone who starts pushing me to do things before I've finished my second cup of coffee can expect me to growl at them.