First, before 9.2 drops, either farm at least 5150 Soul Ash and 1650 Soul Cinders or have a legendary to dismantle in order to craft your new covenant legendary when you unlock it.
Part of the pre-9.2 goals should also be to get all possible relevant Covenants for your class to 80. You can use the previously mentioned Choreghast runs to do that or all the normal ways of increasing renown easily. Regardless, you don’t want to be farming renown in 9.2 because a late change to your class means your level 40 covenant is now your best choice.
If you truly want to min-max, you can also start selecting items from your vaults that have better stats than your dom sockets. For example, if your 252 dom socket item has mastery on it and mastery is hot garbage for your class, go ahead and pick a non-mastery 252 item from your vault in that item slot. Dom Sockets are disabled in all relevant content, including m+. It’s a small bonus, but it’s been 30+ weeks since SoD launched - what else are you going to grab?
You can also go ahead and recraft your legendary item into whatever slot you’ll be using in 9.2 as long as it’s not in a set slot. Many classes are currently using legendary powers in non-optimal slots due to dom sockets. As dom sockets will be disabled in any relevant content at the beginning of season 3, you can get this prep done pre-patch.
Pre-purchase vast quantities of any raiding consumable that you plan on using other than health pots as there is a new tier of health pots coming in 9.2. For the first few weeks after every new patch, consumables skyrocket in cost as people come back to the game and people stop raid logging. You can save a lot of gold by buying consumables now.
The first week of Mythic, you may not want to immediately select an item from your vault. While it may be tempting to grab that item immediately, you can do your Normal and Heroic clears for more set pieces, trinkets, and items in general. You may get a large upgrade that lets you select a different item from your vault. Until you have access to the Creation Catalyst, you should not pick a non-set item in a set item slot from your weekly vault.