if I delete photos from my iPhone to save storage will they still be in the iCloud

No. Deleting a photo will delete it from iCloud and all of your devices.

See Manage your photo and video storage - Apple Support

Hi, so building upon this.. if your pictures are saved on iMac, are you THEN SAFE to delete them off of your phone ? while retaining them on the iMac ?

also is there a way to triple check they are actually saved to iMac / cloud ?

Thanks in Advance for you help :)