The FF upgrade FOMO is very infectious. For a $5000 hard limit starting from scratch, consider the Canon R8 in Ikelite. I cheated by $58 listing the Black Friday Price on the vacuum kit:
Canon R8 body, when on sale, $1,199
Canon RF 100mm macro, every day price, $999
Ikelite R8 housing, MSRP, $1,345
Ikelite Manual Flash Trigger, MSRP, $250
Ikelite Vacuum Kit, Black Friday Price, $98
Ikelite Flat Port for Macro lens, MSRP, $250
Ikelite Extension for Macro lens, MSRP, $220
Canon RF 16mm, every day price, $250
Ikelite Dome Port 16mm wide angle, MSRP, $350
For $1,700 you can add the Canon 8-15 FE later.
Canon EF to RF adapter, MSRP, $129
Canon EF 8-15mm FE lens, upper range of used price, $900
Ikelite Dome Port for FE lens, MSRP, $425
Ikelite Zoom Gear for FE lens, MSRP, $250
