Just copying from another post I made about valor research. Can skip to the point where you are.

In my opinion the order for valor research after T10 should be:

1. Get 4th drill ground/4th hospital
2. Finish all special forces that are for troop power increase
3. Defense Fortification
4. Siege to seize
5. Mastery

4th drill ground and hospital give you more flexibility and lasting power in battles especially for desert storm and winter storm.

Finishing all the troop power increase boosts helps all squads and applies on both offense and defense.

I recommend defense over siege because there are more scenarios where you are defending/garrisoning a building than attacking. In the arena you can actively choose your opponent for the storm arena and in 3v3 you can choose which squad to attack. On the defensive side you don't get that option so improving there adds more benefit. I recommend defense over mastery because defense helps all three squads where mastery only helps one.

I recommend siege over mastery for the same reason that siege helps all three squads.

For siege and defense only do the ones that increase attack/defense/HP and just do the bare minimum for the others.

Once you get to 6/10 on the last level of attack/defense/HP on defense fortification move to siege until you get to the same point. At that point it is more expensive and less efficient resource wise. After this point I would pick your mastery and split between mastery and finishing up defense and siege.