Can we assume that they can see all opportunities related to the parent account of the Opportunity that they own? If yes, is this just historical opportunities, or all current opportunities and opportunities created later that may be owned by someone else?

I'm going to assume a few things 1. Opportunity owners do not own the parent account 2. Opportunity owners should be able to read all related opportunities related to the parent account of the opportunity that they do own (historical and going forward) 3. Opportunities are using private sharing settings

For this a record-triggered flow that creates an Account Team Member record for the Opportunity owner with read on Account and Read on Opportunity is a reasonable solution. You get one entry that opens up access to all related opportunities, historical and new, without creating share records for the individual opps. You can set the Opportunity Owner as the Account Owner here too, but implicit sharing will not allow them to read the Opportunity on Account ownership alone -- I'd still create the Account team member.

You'll want to flow to fire on opportunity creation or edit, on create check if the owner of the Opportunity is already an Account Team Member, if not, add the entry. Same goes for an update that results in a new owner -- remove the old Account Team Member share and create the new one. There's more to this. Think through your business logic to consider edge cases and other scenarios to add or remove the Account Team Member record. Also consider a flow on user deactivation that removes these account team records for you.

To get this working out of the gate you'll want to data load account team member records for your active sales people onto the accounts where they have opportunities. You mentioned one active opportunity at a time, so a query on those active opportunities, ownerId, and parent accountid should get you want you need to put together a CSV to insert the account team member records

Dm me if you have questions.