If you have 10K square sized maps then I think I would do sections of the map in bits. 10K is big but not terrifically big. But it could be very big for a VTT app as opposed to Photoshop and other paint programs. 16Gb is quite a bit of ram and an i7 should be fine. Not familiar with foundry myself to comment how well it might handle large images.

Also, my advice is to save the map in the apps own file format and then export versions for use with the VTT such as JPG etc. Not sure that using webp would preserve all of the special photoshop info in it. Such as layers and colour gamut encoding etc. Only flatten layers when you are sure you want to bake them into one set. But it will lower the ram used to produce the image.

Also, I have said many times, when making any bit of work, name your file as project_01.jpg or whatever file extension it is, then next time save it as project_02.jpg and so on. After a while you have about 10 files you can delete a couple of earlier ones but keep a running set of at least 5 to 10 of them and keep the number incrementing all the time. Then if you have to go back you have the files. Or if something corrupts then in some rare occasions the corruption occurred a few saves ago and you didnt see the problem till later. Better yet, use git or some other source control app but images will eat the drive space with them since they will keep every version.

And then finally, use a USB drive to backup data after a period when it feels icky not to have to do the work again. Better to use an external USB drive than a second directory on the same drive or a second drive on the same machine (as I had a PSU go bang once and took the whole PC with both my drives out so I don't make that mistake again.)

On my VTT you had stamps of variable pixel dimensions and then you stamp them down and it redraws the whole map of all stamps so there was no limit to the resolution or size of map - which was the whole point of it. But most people like to map using a paint package and import the bitmap.

For VTTs best not to put a grid on it because the VTT will do it for you and they wont align. And best not to put on items on the base image which will move about such as doors which you open on the VTT. But you probably know that bit.