1) No
2) CSM Disable make Windows GPT partition boot faster.
3) No. Choose auto it will use PCIe 3.0 x 16 by default.
4) No. Only Install latest NVIDIA WHQL driver.
5) Yes. X299 PCH support RE-BAR. But RE-BAR may not bring massive performance gain.
W11CANARY 26200.5001 Core i9 7980XE 02007108 MCE ME 11.12.95.2499 R6E Modified BIOS 3801 SAMSUNG OG9 FW 1019.0 SSD 970 EVO PLUS 1 TB x 3 NVIDIA RTX 4090 GAME READY 552.22 64GB GSKILL DDR4 3200MHz JBL 9.1 Sound Bar DTS-X