OK some tips to get consistency.
- More Practise.
- When you think you have done enough practise, do more practise just in case.
In all seriousness, consistency is a learned skill, which you will improve at as things progress.
The best way to pick up the energy and speed for new material is to listen to existing material and re-voice it just to get back into the swing of things.
The smaller room, with some acoustic treatment might be a good way to go. It will give you more isolation from external noise. The acoustic treatment can be simple - rockwool insulation on a wooden frame covered in hessian works really well for basic acoustic absorption.
Now, to the microphone, you should be using a condensor cardioid microphone with a basic pop-shield. The SM58 is a dynamic vocal mic which is designed for use very close to the mouth. This is basically how singers will use it. The frequency spectrum of the mic is designed for very close use in a sound-reinforcement context. It is not the right microphone for dialogue recording and will likely be contributing to the problems of consistency you are experiencing as even a very small change in recording distance with the mic will result in a large change in recording level and spectral response.
Something like a Rode NT1A would be a good mic to investigate. You will need to provide phantom power from your desk or interface and would definitely need to use a pop-shield. The sound quality you will get from a condenser such as this will be vastly superior and much more consistent. It will, however be more susceptible to environmental noise (as it is way more sensitive), so work on your room isolation.