The difficulty with these tests is that Expanding Force and Psychic Terrain seem dependent on one another - we're looking at these two modifiers being applied together in the vast majority of cases, but they really should be teased apart to determine their modifiers individually before putting them back together to see if things can be made to work. The first test I did was rule out SadisticMystic's idea that Psychic Terrain + Expanding Force were being combined as a 2x BP modifier:
Test: Level 50 Starmie, 151 Sp. Atk Expanding Force in Psychic Terrain vs. 106 Sp. Def Corviknight, singles
Rolls: 63, 64, 65, 66, 71
63 proved it wasn't a raw 2x modifier, as a 160 BP move couldn't produce 63 given the other factors. Next, I tested Expanding Force + Electrify to remove the Terrain modifier, isolating Expanding Force by itself:
Test: Level 50, 101 Sp. Atk Electrified Expanding Force in Psychic Terrain vs. Magic Powdered Bolthund with 76 Sp. Def, singles
Rolls: 61, 62, 63, 64, 65, 66, 68, 69, 70, 71, 72
72 indicates Expanding Force cannot be either a Sp. Atk modifier or a final damage modifier. It's highly, highly improbable Game Freak slotted the Expanding Force modifier anywhere else, so for now it's safe to say it's BP (though you can trivially easily prove this if needed, probably even with the rolls given already). Next up is locking down Terrain's modifier. Last generation it was a 1.5x modifier to base power, but it was changed to roughly 1.3x.
Test: Level 65, 218 Sp. Atk Volcarona using Psychic in Psychic Terrain vs. 188 Sp. Def Blissey
Rolls: 65, 77
These damage rolls demonstrate that Terrain, if it were a base power modifier, would have to have turned Psychic into either a 116 or 117 BP move, necessitating a modifier somewhere between 5257/4096 - 5347/4096. If Game Freak slotted Terrain where weather is at, it would have to be a base damage of 60 and turn into 77 prior to the random damage rolls, which would necessitate a modifier of 5223/4096 - 5290/4096 (which would be pretty weird, Game Freak hasn't used that range before afaik).
Test: Same as before but with Volcarona at +2 Sp. Atk
Rolls: 134, 136, 139, 140, 146, 149, 151, 153
If Psychic Terrain were a base power modifier, the base damage of this move would be 153; otherwise, if it were Sp. Atk, the base damage of the move would be 154. These are the damage rolls for base damage of 153 vs. 154:
153 base damage: 130, 131, 133, 134, 136, 137, 139, 140, 142, 143, 145, 146, 148, 149, 151, 153
154 base damage: 130, 132, 133, 135, 137, 138, 140, 141, 143, 144, 146, 147, 149, 150, 152, 154
All of the damage rolls line up with 153 base damage, which rules out Terrain as an Attack modifier with certainty. Additionally, if it were a weather modifier, the base damage would have to be 118, as the only possible value to make our damage rolls work is to be 153 by the time we reach the random damage rolls. Going from 118 -> 153 necessitates a modifier range of 5294/4096 - 5328/4096, which has no overlap with the previously established range of 5223/4096 - 5290/4096 and therefore proves Terrain is also not a weather modifier.
Similar math can be run to prove Terrain isn't a post-damage rolls modifier either, just from these two sets of rolls alone, but my notes on this are really bad so I'm just going to assume I did it right. Someone can fact check me if not, but given how consistent my end result is + other values I think it's gonna be impossible to say Terrain is a post-damage rolls mod.
Therefore, we have proved that Terrain is a base power modifier, like last generation. The only thing left is to determine the precise value. We know 5257/4096 - 5347/4096 is the BP range from our +0 Volcarona test.
Test: Level 61 Soaked Kadabra, 165 Sp. Atk, at +6 Sp. Atk using Confusion in Psychic Terrain vs. the same 188 Sp. Def Blissey
Rolls: 105, 111, 112, 114, 116
This narrows the range of possible values from 5284/4096 - 5347/4096.
Test: Same Kadabra using Psybeam in Psychic Terrain vs. same Blissey
Rolls: 146, 150, 152, 153
This narrows the range to 5325/4096 - 5347/4096. This is significant because 5324/4096 is the Life Orb modifier (so we've just ruled that out). Now, at this point it's almost certainly going to be 5325/4096, since that's what the other 1.3x base power modifiers use. But to be certain, we need to provide a distinction between 5325/4096 and 5326/4096. I chose to use Rollout Storage to do so; by using Rollout Storage, you can amplify a move's base power from 2x-32x by manipulating the number of Mimikyu/Eiscue you attack mid-Rollout. For this particular test, I used an Electrified Explosion in Electric Terrain with an 8x Rollout multiplier to give Explosion the distinction of 2600 BP vs. 2601 BP.
Test: level 8 Steelix, 20 Attack, at +2 Attack, with an 8x Rollout Storage multiplier, using Electrified Explosion in Electric Terrain against a Magic Powdered 102 Defense Heliolisk, at -2 Def, single target
Roll: 196
The base damage could have been 205 (2600 BP) or 206 (2601 BP). The distinction is here (I intentionally chose a set of damage rolls with no overlap):
205: 174, 176, 178, 180, 182, 184, 186, 188, 190, 192, 194, 196, 198, 200, 202, 205
206: 175, 177, 179, 181, 183, 185, 187, 189, 191, 193, 195, 197, 199, 201, 203, 206
Since I rolled 196, that means Terrain generated a 2600 BP move, which requires a 5325/4096 multiplier.
For completeness's sake, I went back and redid the Alakazam-Orbeetle test, and got the following rolls the second time:
Test: level 50 Alakazam, 181 Sp. Atk using Expanding Force in Psychic Terrain vs. Orbeetle with 128 Sp. Def
Rolls: 46, 47, 48, 50, 51, 52, 53, 54, 55
Which lines up with the new conclusion that Terrain is 5325/4096x and Expanding Force is 1.5x (technically 6144/4096).