There is some serious padding in that estimate.

It may well cost the $750 to change the timing belt, waterpump and seals on the engine.
But when in there the valve covers are about ¢ hour and $20 parts. He is hitting you real hard there.

I would confirm that you actually have a coolant leak. In 20K you sure should not need a waterpump and if you do I would look into the warranty on the previous one.

Cracked spark plugs, Interesting, does the engine run at this time? It must not if it has cracked spark plugs.

Is your battery bad?

Full tune up. Hoax. Just change your own plugs & wires. Even with ND Iridium plugs and good wires you will have $70 in that. At a slow pace it is an hours work.

The rear brakes last a long time on these. The drums can be tough to pull off if they are rusted on. Out your way they might not be rusted. But for him to claim you need new shoes he must have pulled the drums to check, Right.

All this work and I do not see any mention of greasing the truck. There are many grease fittings that need a squirt.

I would not consider this to be a trust worthy shop.