You won't be racing so 80mm will be too little, but you want good climbing, 125mm can be a bit hairy climbing, 100mm is a good balance. To answer your question, you will be wasting 25 mm by adjusting it to 100, when you can get a bike that has a shock(s) with 100mm.
The more travel, the harder it is to climb.
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Appreciate the old bikes more than the new.