Seattle's starting pitching depth falls off after sixth starter Emerson Hancock, a 2020 first round pick who posted an ERA of 4.75 in 12 MLB starts this year:
https://www.mlb.com/prospects/mariners/
Taylor Dollard, the Mariners' 2022 minor league pitcher of the year, should return from Tommy John surgery while Logan Evans, their top minor league starter this season, is probably a year away.
In the unlikely event Seattle trades an established starter, the M's would likely turn to the free agent market where some pitchers might take a discount to perform in the pitching-friendly environment.
The Red Sox and Mariners each posted a team wRC+ of 104 this season; after August 15 the Red Sox posted a wRC+ of 83 and the Mariners a league-leading wRC+ of 118:
https://www.fangraphs.com/leaders/major-league?pos=all&stats=bat&lg=all&qual=0&type=8&season=2024&month=1000&season1=2024&ind=0&team=0%2Cts&rost=&age=&filter=&players=0&startdate=2024-08-15&enddate=2024-11-01&sortcol=17&sortdir=default&pagenum=1
This season the Red Sox received 20.3 fWAR from position players while the Mariners received 21.2 fWAR from position players:
https://www.fangraphs.com/leaders/major-league?pos=all&stats=bat&lg=all&qual=0&type=8&season=2024&month=0&season1=2024&ind=0&team=0,ts&rost=&age=&filter=&players=0
The narrative that the Red Sox need pitching and the Mariner need hitting is overblown.