This is really interesting thread thank you.
Few things I figured out if in future someone wonders in here.
Trying to add over 3 sockets or sockets to an item that already has sockets causes it not to work at all.
In case you brick your character go into db -> characters and do SELECT * FROM item_instance ORDER BY createdTime DESC LIMIT 1; You can remove bonus id's to get it working again.
Max I could increase item level was with 1672 which increases relics to 1100. (if I remember correctly)
You can increase base item level of relic weapons by deleting it and adding it back with 1672 (which should bring base il to 950), tho I did this after allocating points and I had to do it again.
There is hardcap of 33k (or something like that) for secondary stats, if you fo over they reset to 0.
I noticed that for some reason stats have highest values when item level = 1100, if you go over secondary stats start to go down. With leggys this would be 450:3337:1546 -> 1100il with mythic and titanforged (again if I remeber numbers correctly)
In case you want to add other custom stats to the item here are id's that I found. There are a lot of stuff between them, mostly just different balance with two different stats eg. 100crit and 250mastery or 150crit and 200mastery etc. These listed here are the balanced ones where both stats were equal. If anyone have any more bonues I would love to know.
3 - 2bluesockets + str (not usre if it's always str or it was str just because I was str char)
17 - crit
18 - haste
19 - crit + haste
40 - Avoid
41 - Leech
42 - Speed
43 - Indestructible
55 - crit + mastery
76 - crit + mastery
97 - versa + crit
118 - haste + mastery
139 - crit + haste
160 - haste + versa
185 - haste + mastery
206 - versa + mastery
227 - haste + versa
248 - crit + versa
269 - haste + versa
And ofc if you want you can add same bonus multiple times 17:17:17:17:17:17:17:17:17:17:17, tho I think there might be some limit so don't recommend making anything too crazy. Only time I managed to brick my character was when I added like 15+ into single item.