As has been explained many times, high SoC esp. for long periods of time isn't good for li-ion batteries (except maybe LFP which Bolt doesn't use).Why would fully charging your battery impact your ability to get 300,000 miles out of the battery pack? Please explain with data or science.
If your worried about battery life, dont ever do a DC fast charge. Lv1 or Lv2 to 100% should not impact battery life.
See slide 8 of https://www.nrel.gov/docs/fy16osti/66708.pdf. Notice both cases have charging to 100% but the higher average SoC case (85% vs. 55%) has more degradation.
"*Faster fade at 30% DOD relative to 90% DOD in this scenario is due to longer dwell time at high SOC for the 30% DOD case."
DOD = Depth of Discharge.
I bet the high mileage Teslas you cite weren't sitting at high SoC for very long each day. I bet their average SoC was much lower than a guy who charges to 100%, lets it sit there overnight at 100%, depletes maybe 3 to 10% then charges again to 100%, rinse and repeat.
I've posted What % Should I Charge To? and more in a linked post. It seems like I have to repeat stuff like this pretty much on a weekly basis between here and various Bolt and EV groups I'm on.