deter - I think you're realizing the dirty little secret that many don't recognize, or at least are not willing to admit.
Most often, the "best" cost effective approach to engine lubes is conventional dino oil with bypass filtration; if you keep the dino fluid cleared of contaminants, and top off frequently (as is necessary with TP BP filters), then the life cycle of the fluid is nearly boundless. There is little "benefit" to a synthetic fluid if you keep the dino ultra clean.
The synthetic benefit certainly exists when comparing conventionally-filtered engines, past a certain OCI duration. For example, running a synthetic oil and a dino oil, both with conventional filtration, yields little benefit of the synthetic over the dino for say up to a 7.5k mile OCI. But put that at 15k miles, and the dino would likely be well past it's prime, where the synthethic might still be servicable. The question becomes: if the synthetic costs 2x the money (or more), do you get 2x the serive life to make it viable?
But with dino oils and bypass filtration, the tables are turned. If the insolubles, the wear metals, and the add-pack are all kept at optimum with bypass filtration, then where is the advantage of synthetic? It does not matter how many miles you put on the oil, because the BP filter keeps it all in "like new" condition. And the TP filters necessitate about a quart of top-off at filter change time (every 3k miles or so), so you're getting a huge boost to the add pack quite often, regardless of which fluid base stock you choose.
Given your situation, you might consider the following: dino fluids, TP bypass, and UOA every 10k miles. Do not OCI until the UOA indicates. The cost of a UOA is about $25 (or less, depending upon service used), but an OCI would cost more than that for three gallons of Rotella (perhaps $11/gallon x 3), and you'd presumably be changing the FF filter, for even more cost? I say, stick with the 3k mile bypass filter change, top off as needed, UOA every 10k miles, and oil OCI when needed. I think you'll find huge and pleasantly suprising results from such a plan. You see, in small sump systems, the cost of the UOA is greater than a simple OCI with a conventional fitler. However, larger sump systems with bypass filtration, with longer drain intervals, will always make fiscal sense, because the UOA is cheaper than the OCI. The cost of UOA is fixed, but the cost of oil fill is variable based upon sump size. You are at the point where a UOA is less money than an OCI, so why OCI until you absolutely must do so? The cost of the TP is negligible; the top off quart is only $3 or less, every 3k miles. Why OCI for (at least) $33, when you can UOA for $25 (or less), and likely still use the oil that is in there???
Bypass filters paired with dino fluids are,
without a doubt, the most cost effective method of extending the drain intervals.