Skip the 135.
Have a good adult conversation with them and tell them you are very appreciative of the offer but you respectfully decline because you don’t want to take a job, run up their training cost and leave.
You’d also need a couple of 135 checking events like a 135.293/297/299 that could jeopardize your 121 start if you fail.
If anything get a head start by learning as much about 121 as you can.
Learn about jet engines, systems, regs.
You’ve got 6 months to become the smartest of the class.
Get your ATP with your local DPE.