In terms of performance, both ARMv8 and x86_64 (at least the kind of devices you pointed at) won't have any problems with 300 MBit/s and sqm/cake. Number of ethernet ports is probably easier to fulfill with x86_64 (there are many 4-port x86_64 router boxes).
Given that you just need to cover 300 MBit/s, Tips for getting cheap used x86-based firewall with full Gbit NAT (a PC Engines APU) if you are in the US might provide the cheapest way out (with plenty of headroom for the future).
I've been using OpenWrt on x86_64 for the last 15 months now (switched from an ivy-bridge mini-PC with two ports to a more purpose built 4-port baytrail-d box last summer), with regular sysupgrades (roughly every 2-6 weeks, following master snapshots myself) - and I'm very happy with my decision.
With 300 MBit/s and sqm/cake, you might still get along with mt7622bv (e.g. Belkin rt3200), filogic 830 and probably ipq8072a (e.g. Dynalink DL-WRX36)/ ipq8074a, which would give you internal wireless as well.