Great news that you're satisfied with the E8450 WiFi throughout! Let us know if you try out the E8450 as a combined main gateway router and a WiFi AP, and how it works for you of course. Your E8450 will definitely have some CPU headroom compared to your ER-X SFP.
Coremark loads only one core and it is at best a rough indicator, but my RT3200 benchmarks at 4677 Coremark (same CPU as your E8450) and my retired ER-X at 2224 Coremark (same CPU as your ER-X SFP), using the OpenWrt coremark package run when CPU load is otherwise near zero. For grins, my EA8500 (1.4 GHz ipq8064) yields 5715 and my RT4230W (1.725 GHz ipq8065) yields 7029 Coremark - these "ancient" mid 2010's Krait CPU's are not so old and tired - but an A72 core on my Nano Pi R4S still takes them at 10,500 Coremark.
For your future gateway specs, yeah you're probably looking at x86. Another option you may want to keep an eye on - it is not supported by OpenWrt yet, but may be in future - is the NanoPi R6S.