3.8 million points in Scrabble

Question

You are playing English Scrabble solo, but on a 15×15 board that looks like this:

The starting square is the top-left corner and all 225 squares are quadruple word squares. The game ends when you place all tiles on the board, and you can choose your rack's contents as you want from the remaining, unused tiles at every turn. All other rules are the same, including the 50-point bingo bonus.

Can you score at least 3.8 million points using this board and the CSW19 lexicon? (After you do so, what is the highest score you can get?)

_{This puzzle came about while I was playing with Quackle and its ability to handle custom boards.}

Answer 1

I was able to come up with this:

Move	Score
1. 1A HYDROXY	393266
2. H1 LOCKJAW	376982
3. A1 (H)OSPITAL	213042
4. A1 (HOSPITAL)IZATION	475186
5. 1A (HYDROXYL)APATITE	557106
6. O1 (E)THEREAL	180274
7. O1 (ETHEREAL)ISATION	294962
8. 12H RUNABOU(T)	163890
9. 3D VERE(C)UND	229426
10. 7D UNCO(W)LED	229426
11. D5 SQ(U)iDGES	294962
12. 15H EMBIGGE(N)	229426
13. M1 (I)NFIRMER	213042
14. 10H WAVEOfF(S)	262194
15. H10 (W)I(R)E	112
Total	4113296

Of course it's critical to always play 7 letters for the 16384x point bonus - the whole problem is about making those 14/15 letter words (I got 3, it's certainly possible to make 4 or 5+) and re-using high-value letters in them, which I only did well with my X... So it's for sure possible to get 5 million, but this is already nice enough.