I would definitely go with the 5”

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I would talk to the manufacturer, and I'm sure they'll reassure you, assuming you're considering engineered wood, which is designed for dimensional stability.

Low humidity is probably worse than high humidity. In high humidity areas it's generally hot so people use HVAC to cool the house, bringing the humidity down to a tolerable level. Even without HVAC wood floors can withstand high humidity. It's the seasonal change in humidity that often causes issues. Manufacturers generally recommend the humidity be above 35%. Below that, the wood can shrink and crack. In dry areas people must install a whole home humidifier (and keep it maintained) to avoid dry cracking. This can happen to solid wood and engineered wood. The same thing is true in hot, dry areas and very cold areas where the humidity drops to zero in the winter. Your local floor dealer should know this. Manufacturers should know this but they may not know your local conditions and they will take no responsibility if something goes wrong, regardless of how they may advise you now.

Wider boards aren't an issue as long as your floor is acclimated properly and the humidity doesn't fluctuate too much. Solid boards over 4" wide are more susceptible to cupping and the wider the board, the greater the potential to develop gaps between boards.

In a 5" plank width I would highly recommend engineered products. You will have less squeaking, cupping and gapping over the year. We have a pretty straight rule at our company that we do not install solid products over 4.5" wide. We want the customer to be happy with their purchase and unfortunately with a solid plank over 4 or 4.5" there's just too many issues that we've seen occur in the past.

You want to buy engineered or you want to buy problems. Thems your choices.

You need a home humifification system regardless. Even engineered will not have a warranty at 12% moisture.