In general, look for mt7621 (Xiaomi mir3g, ZBT WE1326, ZBT WG3526, AFoundry EW1200, ...), ipq40xx (AVM Fritz!Box 4040, ...) or entry level mvebu (Linksys WRT1200AC); if you're looking for r7800 like performance, the ZyXEL NBG6817 is almost identical (more flash (4 GB eMMC), but no SATA), but can often be found a little cheaper than the r7800.
Check the individual device pages before buying, for mt7621 I've only listed popular mt7603e+mt7612e devices, for ipq40xx I've only listed devices with more than 128 MB RAM (ath10k needs RAM) and at least 16 MB flash.