Any slack would be in the boxes at each end of the conduit.

You do not really need much though.

As long as the cable isn't pulled unreasonably tight, there should plenty of room for thermal expansion.

It's not the thermal expansion/contraction that you have to
worry about. It's the settling of the trench, (I've seen
meter boxes ripped right off of houses for this reason).
But if your trench has been there for a while, it will
probably be alright.

I'd like to throw up a small flag since you are using aluminum wire...I'm sure you are aware but I thought I'd mention to make certain you use the proper method for terminating aluminum wire or your temperature swings will certainly have an impact.

"I'd like to throw up a small flag since you are using aluminum wire...I'm sure you are aware but I thought I'd mention to make certain you use the proper method for terminating aluminum wire or your temperature swings will certainly have an impact."

Yes, aluminum woire needs to be correctlt terminated.

There has not been any issue with correctly terminated Al stranded wire in larger sizes.

The issues have been with single solid conductor aluminum wire, and there is NO WAY to correctly terminate these.

The Al is not tempered hard enough to allow for installation and handling, but this then allows it to cold flow under screw heads and other pressure type connectors and become loose after repeated thermal cycles.