Thank you so much for the follow-up response @v-yueyun-msft! I really appreciate your time and efforts!
Here is my main list (ListData2) where users create an item for signature/approval.
Signers is a dynamic multi-select person column.
Assigned To is a single-select person column and automatically gets the first signer then moves to the next one in the sequence.
Signatures Collected (default value = 0) column counts the number of actions taken by the approvers. Flow continues to forward the approval until Signatures Collected = number of signers, unless anyone rejects.

Here is my second list (ListData2Delegation) where signers/approvers log their time away.
Delegating To is a single-select person column. 'Created by' will be one of the approvers that assigns a backup for a period of time (From, To).

My Approval flow type is Approve/Reject - First to respond because I need the signers to sign in order.
I need help to update the 'Signers' column through a Filter query from the ListData2Delegation
where > (Author/Email eq 'Current Signer or Current Assigned To') and (From le 'utcNow()' ) and (To ge 'utcNow()').
Replace the current 'Signer' (when it's his/her turn) with assigned back up from ListData2Delegation and apply the same to other Signers if applicable.
Here is my approval flow without the delegation part. Not sure how to properly execute it. The screenshot from my first post is a test flow I'm trying.

Thanks in advance for sharing your knowledge! Greatly appreciate it!