It doesn’t matter whether it’s charged externally or internally. The battery is fine with 4.35V, but you shouldn’t put a battery charged above 4.2V into the phone at all. The internal charger is already configured to not go above 4.2V, while when charging externally it’s up to you to ensure that it doesn’t go above 4.2V.
If you do happen to charge it above 4.2V, you should discharge it a bit before putting into the phone so it won’t provide voltage higher than 4.2V to the phone.
(as already noted somewhere else, the phone should actually be fine with 4.35V, but IIRC one of the M.2 cards is not; plus I haven’t checked the whole power path to be sure that there are no other components that won’t like voltages above 4.2V, so can’t do any claims about that either)