Transaction isolation level/design for write-only ledger

This is a simplified example of a problem I've been working on. Say I have the following database schema:

-none
Table: Deposits
+--------+------------+--------+---------+
|   ID   |    Date    | Amount | User ID |
+--------+------------+--------+---------+
| c1f... | 1589993488 |   40.0 | 6c7...  |
| bfe... | 1589994420 |   30.0 | 744...  |
+--------+------------+--------+---------+

-none
Table: Withdrawals
+--------+------------+--------+---------+
|   ID   |    Date    | Amount | User ID |
+--------+------------+--------+---------+
| ad4... | 1589995414 |   20.0 | 6c7...  |
| e9b... | 1589996417 |   20.0 | 6c7...  |
+--------+------------+--------+---------+

And I'm writing a function that **performs a withdrawal** for a User. It: 1. Sums the deposits (SELECT amount FROM deposits WHERE user_id = ...) 2. Sums the withdrawals (SELECT amount FROM withdrawals WHERE user_id = ...) 3. If the difference is greater than the requested amount, INSERTs a new Withdrawal We're using MySQL 8 and the default isolation level of REPEATABLE READ. This function may be running as a lambda, so memory locks are not an option, and we don't have distributed locking (ie. a Redid-based lock) available. **A Caveat** There are existing admin operations run at the REPEATABLE READ level to create/delete these entities on-demand, by ID. **My Questions:** 1. Am I correct in understanding that I need to use SERIALIZABLE as the isolation level for this transaction? 2. Will the SERIALIZABLE range lock prevent the REPEATABLE READ transaction from inserting new rows into the Withdrawals, or removing rows from the Deposits?