PFB the class and VF page for send email functionality, you can use this to extend it your opps. You might need to make changes to use it for your custom functionality. Hope this helps!

Apex Class

Public class SendEmail
{
    public String subject {get; set;}
    public String body {get; set;}
 
    private opportunity opp;
   
    // Constructor to populate instance of your object
    public SendEmail(ApexPages.StandardController controller) {
    this.opp= (opportunity )controller.getRecord();
    opp= [SELECT opp fields required
             FROM opportunity
             WHERE id = :ApexPages.CurrentPage().getParameters().get('id')];
    }
             
    public Opportunity getOpportunity(){
    return opp;
    }
   
   
    public pageReference send(){
    Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();  
     
    email.setSubject(subject);
    email.setToAddresses(address);//Use SOQL to retrieve addresses in the address
    email.setPlainTextBody(body);
    email.setBccSender(true);
   
    Messaging.SendEmailResult [] r = Messaging.SendEmail(new Messaging.SingleEmailMessage[] {email});     
    for ( Messaging.sendEmailResult result : r ) {

           if ( !r[0].isSuccess () ) {
              
               System.debug ( result  );
           }

           else{

               ApexPages.Message msg = new ApexPages.Message(ApexPages.Severity.INFO, 'Email Sent Successfully' );
               ApexPages.addMessage(msg);
           }
       }
    return null;  
}
}

VF Page

<apex:page StandardController="Opportunity" extensions="SendEmail">
<apex:pageblock title="">
<apex:pageMessages ></apex:pageMessages>
<apex:form >

<apex:outputLabel value="Subject" for="Subject"/>:<br/>
<apex:inputText value="{!subject}" id="Subject" maxlength="80"/>
<br/><br/>
<apex:outputLabel value="Body" for="Body"/>:<br/>
<apex:inputTextarea value="{!body}" id="Body" rows="10" cols="80"/>
<br/><br/><br/>
<apex:commandButton value="Send Email" action="{!send}"/>
</apex:form>
</apex:pageblock>
</apex:page>