The Chargers were #2 in pass attempts (711) last season. New Chargers OC Kellen Moore was the Dallas OC last season, and the Cowboys were #19 in pass attempts (556). I'm sure the 2023 Chargers will pass more frequently than 2022 Dallas, since the Chargers passing game players are better than the Cowboys' equivalents, but I seriously doubt they will have 700+ pass attempts again. I think 650 is much more likely... which would mean fewer targets to go around.
650 is reasonable, but the ADOT will increase as well offsetting the target difference...to what degree we'll have to find out.
QJ is the X factor, how quickly he can acclimate will have a lot to do with Williams' volume IMHO.
I agree ADOT should increase, but YPA obviously matters more. So far in his career:
2020 - HC Lynn, OC Steichen - ADOT 7.8, YPA 7.3
2021 - HC Staley, OC Lombardi - ADOT 7.9, YPA 7.5
2022 - HC Staley, OC Lombardi - ADOT 7.0, YPA 6.8
Now he has Moore as his OC. In the 4 years he was the Dallas OC, here is how Dak compared:
2019 - ADOT 9.8, YPA 8.2
2020 - ADOT 8.3, YPA 8.4
2021 - ADOT 8.3, YPA 7.4
2022 - ADOT 8.6, YPA 7.3
That was on lower volume - 36.9 attempts per game for Dak, 40.1 for Herbert.
I expect Herbert's ADOT will be somewhere between 8.0 and 8.5. If so, his YPA should be a career high, maybe 7.8, give or take. That would be 5,070 passing yards on 650 attempts. That would be a new career high for him and a nice improvement over last season, so you are right that it should offset the reduced attempts.
As for QJ, all reports have been positive so far, though we are obviously in fluff season, so hard to take anything at face value.