This would entirely depend on the information available from the dedicated membership system.
I would consider that your current setup is an overkill in that you are virtually duplicating the membership system (per member) into Manager via the member special account.
Look to see if the membership system has some form of statistical output - unexpired subscription periods, if yes, let the membership system be the detail and Manager be the global perspective.
Depending on the data available will determine how you can calculate a valuation for the unexpired subscriptions.
Base Level - Member A - 10 (days to go), Member B - 125, Member C - 67 etc, etc.
Accumulate these and multiple by a subscription factor* - therefore at any month end your Manager “Gym Memberships” liability account would equate to this valuation, and the Journal would transfer to the P&L any adjustment between two valuations of the unexpired subscriptions.
Noting that the liability account would also be receiving “additions” from new subscriptions.
At a higher level - if subscription periods were also available then this would enable the calculation of an even more accurate subscription factor - if the subscription period members were grouped (sorted) together (24 mth,12 mth) then valuations per period could be used.
Unless this dual process is really required then this to my mind is also a duplication - Sales Invoice + Receive Money. I am assuming that most subscriptions are payment up front (not on credit) therefore the Cash Sale entry should suffice along with its produced receipt. The setting up of members as Manager Customer’s is a duplication of the Membership Customer
If the membership system produces a renewal notice then use that as a pro forma invoice.
Even if you want to maintain the control account + special account structure, using the Cash Sale will also “very nicely prompt you to select the created Gym Member special account”
*Subscription Factor
The basis for this calculation will entirely depend on the statistical data available.
At the base level assuming that the different contract periods will have different monthly rates, the subscription factor would be a weighting between them.
3mth - 30/mth, 12mth - 18/mth, 24mth - 12/mth = 20/mth.
This factor is multiplied by the number of periods of unexpired subscription. Then once or twice a year do a full (member x member) valuation - just like a stocktake.
The better the statistical data available the more accurate the valuation of the unexpired subscription.
To transfer the unearned income stored in the Balance Sheet to the Profit & Loss as it becomes earned.
Exactly the same as an insurance company does with unearned premium income.