Player file format changes w.r.t. format 102/v1.2.4.1:
After the game version 32bit int, skip/read the first new unknown block of 20 bytes.
Continue as before reading the name bytes.
After the is-male boolean, read 10 bytes.
Continue as before reading the health bytes.
After the buffs section, read 143 bytes.
Continue as before reading the world bytes.
At the previous end of the file, after the count of angler quests finished, read 13 bytes.
This should get you able to read the 146format/v1.3 file as though it were the previous, and ofc by saving those 4 byte chunks and writing them in the same sequence in an output file, you can create a valid 1.3 file that the game seems happy to load.
:-D
Now to work out what those 4 chunks contain...
edit: ok that's not quite right, noticed it's mis-aligned for armour and a bunch of other things. I think I need to shift some unknown bytes to be before the armour section...
edit 2: there's a 5 byte unknown somewhere between mana max and first armour item. This fixed my armour index alignment.
And there's still something more serious making my 1st calculated inventory slot still fall inside the inventory, but the 10th not...
Proper effort required tomorrow it seems.