Maybe this is a graphics card problem, when installing Elementary OS did you select to install proprietary drivers?
When you use Nvidia graphics it is advised to do so, to install such drivers during the installation just select the box that says
Install third-party software for graphics and WI-FI hardware and additional media formats
Installing eOS on a 2015 MacBook Pro is great, as nearly everything works out of the box. I recently installed eOS 6 beta on a 13-inch 2015 MacBook Pro, and it works great. Dual-booting is also a great choice, as it makes updating firmware easy if/when necessary.
Since elementaryOS is based on Ubuntu, guides for dual booting Ubuntu should be directly ...