Material to fill a 2-meter deep space under the new terrace

Would you have it filled easily for €2000 + VAT or do it yourself in three days with about €1000 savings.

You can only answer this question yourself. Ultimately, it can even make sense if you only save €100. Everyone has their own limits.

Load, unload, and compact 25 tons of material.
That's a lot of wheelbarrows, I would roughly estimate 200-300 trips.
By tamper, do you mean one of those hand tampers?
Have you done that before? It's very hard work, your arms will get heavy after a few minutes.
Do you have space for 25 tons of material? That's quite a pile.
If you have 2-3 strong guys to help you and you make a nice barbecue evening out of it afterwards, do it yourself. If you're alone, have it done, otherwise you'll curse yourself after the first day.

I also believe that three days are not enough, but I would probably do it myself.

If a pile is lying, a small wheel loader is best. Drive the shovel underneath, lift the full shovel (about 2 wheelbarrows worth), drive to the destination, dump, level off, and repeat. If it goes into the container, definitely have it dumped, 1 day is enough.

Doing it yourself would mean having the material (2 dump trucks each 12-13t) dumped into the yard. Renting a Bobcat with a bucket and then always driving it to the hole. The Bobcat can carry 400l per trip. In the hole, someone would need to compact every 30cm with the rented vibrating compactor. This would be done layer by layer until the hole is filled. Material and rental costs for 3 days would be about €1200. I will, however, speak with the construction company again to see if I can reduce the costs (€2000+VAT) a bit.

With a Bobcat and a proper vibrating plate, it’s already a completely different matter. It can be done in one day if there are two of you. By the way, your 25t is clearly too little. According to an online calculator, you need almost 35t of recycled material for 2m x 2.5m x 3.5m with 15% compression. Even without compression, it should be almost 30t.