Deserialize JSON to Object given an JSON String and an sObjectApiName

Question

I created a generic LWC using a lightning-record-edit-form where user specifies sObjectName and fields to display as parameters and the edit form will display those.

<lightning-record-edit-form
           record-id={recordId}
           object-api-name={objectApiName}
           onsubmit={handleSubmit}>    
<template forEach={field} ...
<lighting-input-field ...>

When user clicks on submit, I would like to call an apex class and have that class deserialize the form into a specific object.

For example: on an app page, I configure my form to objectApiName ="Contact" and Fields = "FirstName, LastName"

I would like to imperatively call an apex class (pass whatever paramaters needed from JS) and within that class I would like to generically be able to deserialize my JS parameters into an sObject.

currently my event handler looks something like this:

handleSubmit(event){
    event.preventDefault();
    const fields = event.detail.fields;
    let strFields = JSON.stringify(fields);
    doBeforeSave{objectApiName: this.objectApiName, jsonFields: strFields}
    .then(()=>{...})
    .catch((error)=>{...}

If I know this is a Contact for example, in my Apex method I can do something like the following:

Contact c = (Contact)JSON.deserialize(strFields, Contact.class);

If my LWC was configured for an Account with Account fields, this would fail.

If I try to deserialize this the following way I get an error:

sObject myObj = (sObject)JSON.deserialize(strFields, sObject.class);

Nested object for polymorphic foreign key must have an attributes field before any other fields. So I would need an "Attribute": {...} appearing first, but I do not have that in my event.detail.fields object. See this question

I found a way that works by instantiating a new sObject, deserializing the jsonFields into a map<String, Object> and then putting these in the sObject:

sObject genericObj = sObjType.newSObject();
Schema.SObjectType t = Schema.getGlobalDescribe().get(objectApiName);
SObject sObj = t.newSObject();
Map<String, Object> fieldMap = (Map<String, Object>)JSON.deserializeUntyped(jsonFields);
for(String fieldName : fieldMap.keyset()){
    sObj.put(fieldName, fieldMap.get(fieldName));
}

but this does not seem optimal.

Any help on best way to:

• given two strings: objectAPIName and jsonString of fieldName: fieldValue, how can I generically deserialize this into a typed sObject?

Thank you for your suggestions