Is there something I can do at home simply rather than go to a workshop or should I just fit the upgrade and see?

just install the new bb and you will have no issues. refacing the shell is a professional service and i would trust only a handful of shops to do it well. also proper tools are expensive and if your shop took a shortcut there, you are fucked

So in the end rather than having perfect angles, you probably have to deal with a ruined frame.

On the other hand, you will never notice a difference. headset bearings will last a little longer, brakes will brake a little better, and bottom bracket will be a little more efficent and last longer...little benefit, huge cost if done right