Circling back on one of my requested features for fusing cards in combat, I came across three plugins that mimic what I am hoping for. The first two will require MV or the Project Fossil Plugin for MZ. I'm posting these links for idea-building purposes.
1). Rune Skills: Rune Skills (Skill System) | RPG Maker Forums (rpgmakerweb.com) This system allows you to create rune skills that can be combined in the battle to create powerful skills. I can't seem to get it to work properly and the skills only appear as skill icons, not cards.
2). Victor Engine: VE Mix Actions: Mix Actions | Victor Engine (wordpress.com) This plugin allows you to combine skills, items, and/or skills and items in battle. This plugin is very flexible but requires a lot of setups as you have to manually set up different actor commands for the skills and items you want a party member to be able to mix. I got this one to work but it is very buggy as sometimes while I am mixing actions, it will use a card instead of the mixed action. Like the Rune skills plugin, it is a skill icon, not a card when selecting what skills to mix.
3). Battle System - BTB VisuStella MZ - Yanfly.moe Wiki This battle system uses the boost points (BP) like in Bravely Default and Octopath Traveler to use more than one action. However, this system has an action fusion feature that allows you to use boost points to use multiple actions and depending on the actions, you instead unleash a more powerful skill. For example, you use 3 BP to use three random skills. Let's say you use 3BP and use it with three Attack cards. Instead of three attacks, you activate an action fusion skill called Triple Attack. This plugin isn't as flexible as the VE Mix Actions because this one doesn't let you mix skills and items. This one also requires a lot of setup. You can only combine skills with skills and items with items. I can't use this plugin because it isn't compatible with this card battle plugin. I'm not sure if it will show up as cards or skill icons like the prior mentioned plugins.