I'm not sure what it was, but I was definitely missing something. I went back to the old code and edited it adding a new fund number and it deployed fine. Trying to change it to dynamic apex wasn't working for my test class.

Now I'm hitting a too many code statements in another class. Do you see a better way I write the mapping in this code?

public class Trades_CascadeAccounts {
    public Trades__c[] tradesOldList { set; get; }
    public Trades__c[] tradesNewList { set; get; }
    public Map<Id, Trades__c> tradesOldListMap { set; get; }
    
    public Trades_CascadeAccounts(Trades__c[] tradesOldList, Trades__c[] tradesNewList) {
        this.tradesNewList = tradesNewList == null ? new Trades__c[0] : tradesNewList.clone();
        this.tradesOldList = tradesOldList == null ? new Trades__c[0] : tradesOldList.clone();
        this.tradesOldListMap = new Map<Id, Trades__c>(this.tradesOldList);
    }
    
    public void execute() {
        Set<Id> tradesAccountIds = new Set<Id> {};
        for(Trades__c tradesNew : tradesNewList) {

			Trades__c tradesOld = tradesOldListMap.containsKey(tradesNew.Id) ? tradesOldListMap.get(tradesNew.Id) : new Trades__c();

			tradesAccountIds.addAll(new Set<Id> { tradesOld.Resolved_Firm_Trading_ID__c, tradesNew.Resolved_Firm_Trading_ID__c });
        }
		tradesAccountIds.remove(null);

        Account[] accountList = new Account[0];
        
        for(Id accountId : tradesAccountIds) {

			accountList.add(new Account(Id=AccountId,Rollup_Trades__c=null));
        }
        
        update accountList;
    }
}