I was planning on making a warlock under hexblade, and it says you can use the shield spell, which is nice, but it takes open hands. In the description for pact of the blade, it says dispelling the pact weapon is a thing you do without an action. My understanding is that you could have the weapon, and, during the reaction, dispel it and then cast Shield. This is specific to my circumstances, but I feel like there are probably similar things for other classes. I just want to get the general consensus on this idea.

Yes the Shield spell has a Somatic component which means it requires a (singular) free hand to cast. So unless you have both hands full with your weapon in one and a shield in the other - or two one-handed weapons - you can cast it just fine. A two-handed melee weapon doesn't require two-hands to hold it - just to attack with it - so you would be able to cast with one of them too.

If having both hands full is an issue - get the War Caster feat specifically for the second bullet point:

• You can perform the somatic components of spells even when you have weapons or a shield in one or both hands.