Rain head means what it implies......... a fairly LOUSY shower. The only way to keep that window "dry" is a second shower curtain, water proof,,...on a tension rod at the back wall.

Waterproof window curtain. Had one years ago.

To put a shower into this bath area you will need to put waterproof material on the upper walls sealed to your tile as well as into and around the window being sure to seal it all. OR put in a full tub surround again sealing it around the window. You could use a full wrap around shower curtain that goes all the way around the tub, over the window and over all the walls may work temporarily but there would be moisture damage eventually.

Window is new. Is it vinyl? Was the opening flashed on the interior side like the exterior so it will not be effected by water? If not a shower curtain in that side is about your only option, shy if losing the window which you already said was not happening.

A waterproof window curtain, not a shower curtain that will hang on the inside of the tub. Yes to tile all around. I love my rain head shower, but I’ve also been in less than ideal ones so shop carefully to get one to meet expectations.

Are you going to gut the bathtub area to add a new shower? If so, it would be much better to replace the new window with one that is much higher up on the wall. A wide window that is only a foot or so tall could be out of range of the shower spray when installed near the ceiling.

That was not designed to be a proper tub shower. You cannot retrofit it without starting over. I think I even see some electrical where it should not be! The only way that you don't rot out your house is if that is vinyl or fiberglass, and you have someone that knows what they are doing doing the waterproofing and tiling, after you gut all the rest and start over. You will need to seal a sheet membrane to the vinyl or fiberglass and any interior window stool will need to slope back into the tub. This isn't a job for the desperate guys in your neighborhood FB groups. You need someone with experience, education, and a license, if your location requires that. https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer

How about we start with how extensive is this project? Are you doing a full renovation? If the window is installed properly and is vinyl, you can put tile all around it and a sill piece out of stone that pitches slightly forward for drainage. But someone must check if the window is done properly. The wall tiles and sheetrock need to come down and cement board installed after the plumbing is fixed and whatever that is on the wall to the right of the window (??) is removed.

Plenty of people have windows in their showers.

The rainhead - now that's another story. Had one- hated it . Not enough pressure and not enough angle. To each his own.

Good luck!

You can find waterproof curtains to protect your window.

We’ve had so bath/shower combos with a window. In both cases the window was waterproofed and tiled. Never had a single issue. You do need to tile/waterproof far more than show in your pick