I'm seeing two options. As you probably know, lithium-ion batteries experience more aging when stored at full charge compared to when stored at partial charge, but cycling them also wears them out. I don't know which would actually be better:
(A) Leaving the external battery turned on all the time so the phone battery stays fully charged, meaning it will experience more aging but less cycling wear.
(B) Trying to follow a specific cycling strategy, meaning less aging but more cycling wear.
If you are using the phone really heavily, meaning you'd accrue a lot of cycles over a given time, my gut is to go with A. If you'd normally get over a day between charges, I might be tempted to try B.
If you choose a cycling strategy, generally, if you can keep the battery voltage in the middle of its normal range, you should see the best cycle life - so perhaps start at ~3.3V and charge to to ~3.9V.
Gauss163 has shared a paper a few times that shows a handful of tests regarding this. If I remember right, compared to cycling batteries from 0-100% state of charge, or even 50-100% SoC, they saw a significant increase in total energy delivered (not just cycles) over the life of the cell by cycling from 25-75% SoC. I've seen other sources recommend a 20-80% cycling range. I don't know if this is related, but I think 75-80% charge is usually right at the edge of where charging switches from constant current to constant voltage. It wouldn't surprise me if cycling wear increase somewhat at that point.
Can you reliably control the charge this way, however? I guess since smartphones have their own battery meter that I think works mainly by Coulomb counting, you hypothetically could know SoC with decent accuracy directly, instead of relying on voltage as a proxy for SoC. Every now and then (maybe 100 cycles or so?), you'd probably want to do a full discharge until you get a low battery shutdown warning and then a full charge in order to recalibrate the meter against drift and capacity loss. I'm assuming low voltage shutdown on the S8 is based on voltage.
Is it worth the effort of the extra attention required, though? I suppose it's just something you could try to get in the habit of doing when you think about it, and not worry too much if you forget, because at least for those times you remember, you're managing the battery life better than most people.
It would be harder to control the cycling of the external battery, but even if you don't get comparable life out of it as out of your phone, it's at least less expensive to replace, or you just keep using the phone without it.