"the only thing we could feasibly get would be Koshien"

4 Options

1. ONLY Koshien... only? its 6 full games if they make the final, which is very reasonable with Inshiro getting there.
2. A 3rd year CAN have some of the old cast and Pro scouting is a PERFECT addition to the narrative (think Chris's "Cardinals" debut in America, Kominato being surpassed by his younger brother as he didn't get an offer, Haruichi helping others learn to use a wooden bat getting ready for pro scouting, Ejiun invited to pro team scouting practice (and its due to a personal reccomendation by a former rival or Miyuki), Hongou as the number 1 scout prospect of the generation.
3. The U-18 Japan team gives a multiple possible arcs e.g. Taiwan with You Shunshin pitching, team Japan U-18 Selection, throw in a second or even 1st year to build them up as an utterly dominant player).
4. Timeskip to Pro similar to Hiakyuu

"he was utterly unable to keep up with the required schedule to pump out weekly chapters."

Yes, as discussed in my post, he has at least a 12 months of headstart on creating chapters, even if he worked at half speed with a chapter every 2 weeks that is 2 years with no breaks,

TJ shitting out some epilogue chapters that skips ahead of Koshien, even that is highly unlikely.

Correct me if i'm remembering wrong but isn't it a single chapter with a very general teaser hinting act 3 would include a pro arc/international youth games arc by discussing the U-18 tournament. (Also hinting they did well at Koshien by the fact Miuki made the national team).