Making pie or crisp out of thawed fruit that is mostly juice

You should include the liquid and account for it in measuring your thickener.

Remember: fruit is mostly water. Take a look at a chart listing fruits by water content. Berries, stone fruit, and your other likely pie candidates all hover around 85% water; most of it escapes while the pie cooks. So to get a thick pie filling, you need quite a bit of sugar and starch.

When fruit is frozen, ice crystals break up some of the structure of the constituent cells. When it thaws, the fruit will be quite soft and juices may escape. However, no new water is introduced. Thus you need to retain the juices to get the same overall water content as fresh fruit.

I would caution against thickening without any measurements though. To get consistent results, you should really be thickening by weight. I follow these instructions from Stella Parks: she thickens fruit with 5.5% of its weight in tapioca starch and 25% of its weight in sugar. Cornstarch is a slightly stronger thickener; consistent with this chart from King Arthur Flour, you should reduce the amount of thickener by a third.

Thus my final advise is:

1. Weigh out the berries and any escaped juices.
2. Multiply the weight by 0.25 and add that much sugar.
3. Multiply the weight by 0.037 and add that much cornstarch.

Cornstarch sets at 203°F (95°C) so make sure your pie filling is bubbling before removing it from the oven.