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I would advise feeding multiple times per year, or change to a stronger granular slow release. Check the pH, as well.

Make sure to watch the watering. It is a delicate balance of keeping the top of the soil slightly moist, without it being too wet.

These articles will help:

https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/fertilizing-hydrangeas.htm

https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea

They look like panicle hydrangeas which should be cut back in spring, if needed. Panicles bloom on new wood, which means the current season's growth. They have conical shaped blooms, often white, fading to pink. You can cut back up to one-third of the limbs.

If they were mophead hydrangeas, which usually have much bigger leaves, you would trim them immediately after flowering (they bloom on old wood). You can examine the ends of the limbs and if you see buds forming, wait to prune them till after next year's bloom. They have big, round balls of blooms.

If you are nervous about the variety you have, don't prune at all.

https://extension.umn.edu/yard-and-garden-news/when-prune-hydrangeas-best-bloom

That is Hydrangea paniculata! The Panicle Hydrangea.

https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/common-hydrangea-varieties.htm

https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/tree-hydrangea-growing.htm

Yes, here in the Metroplex, it gets quite cold during January and February and a pot's material will not provide cold protection unless the pots are pots that are buried in the ground. Potted hydrangeas in pots should use rollers and then you can easily move them into the garage but, do not forget to water the pot now and then (when dormant, every two weeks maybe?). If temperatures are going to drop considerably like they did in February 2021, consider bringing the pots into the home until the temperatures warm up a tad (then move them back into the garage).

No, most Hydrangeas cannot tolerate full sun. They need a good bit of shade to keep it happy.

Unfortunately, this is the plight of using a container that doesn't have adequate drainage. It is best to have a container with holes that allow all extra water out, freely. Potting into a pot with adequate drainage holes, and letting it dry out some will be the next step.

A fungicide can help prevent damage from the root suffocation:

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

This collection of articles will help you care for Hydrangeas:

https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea

Depends on the type of hydrangea (see the link below). Normally, macrophyllas (aka, Big Leaf Hydrangea, French Hydrangea, Hortensia Hydrangeas), serratas (aka, Mountain Hydrangeas), quercifolias (aka, Oakleaf Hydrangeas) and anomalas (aka, Climbing Hydrangeas) produce invisible flower buds for spring 2022 at the ends of the stems by the end of summer 2021 or the start of fall 2021. See the link below to determine which type you have. Note: if the above named types of hydrangeas typically lose all stems during winter in Marengo, you could prune the stems now all the way down. However, pruning normally triggers new growth that may get zapped by early frosts (the average date of first frost in Marengo is the first week in October) so, I would prune after the plant has gone dormant or at the end of winter.

If your hydrangea is an arboescens (Smooth Hydrangea or Annabelle-like Hydrangea) or a paniculata (aka, Pee Gee Hydrangea), you could prune those now since they will not produce flower buds until 2022. Hydrangea arborescens develop flower buds from mid spring to early summer (it depends on one's geographical location; earlier in the south and after in the north). Hydrangea paniculatas similarly produce flower buds a few weeks later than Hydrangea arborescens. Both can be pruned now or later but avoid pruning from mid spring 2022 to summer 2022.

You can deadhead (not the same thing as pruning) spent flowers at any time by cutting the peduncle string that connects the bloom to the stem.