I'm not sure you can provide the best environment for your FLF to thrive. Direct outdoor sun is much too intense for it especially since it will have to go back inside once temps drop below 50 degrees F. Outdoors it should be on the north side of a building or under something that will keep it in shade throughout the day.
From your description, the light available indoors is minimal at best. In reduced light, your FLF will not be able to support as many leaves. It will continue to add new leaves at the ends of each stem, but it will sacrifice lower leaves to compensate. It will not die inside, but it will struggle and thin out and become leggier.
This is a tropical species plant that does not have a dormant period and requires consistent care year-round. So making it go dormant is not the answer.
Do your best to provide as little outdoor sun when it is outside and as much indoor sun when it is inside.
The best soil for your FLF is the soil it is now growing in. You cannot improve on it regardless of what you may read online. Leave the roots and soil undisturbed.
Water your plant as soon as the surface of the soil feels almost dry and water it thoroughly until some water runs through the drain holes. After it is indoors, allows the top half inch to dry before watering it.
(My information is based on my care of over more than a hundred FLF's indoors in a variety of locations.)
