There are no good USB wireless cards suitable for AP mode and 24/7 operations. Performance and range would be horrible thanks to the tiny antennas (which tend to be too close to each other for MIMO to be effective as well).
Have a look at second hand wireless routers, sometimes you can find incredibly cheap offers with good specifications. The ipq806x range of devices would be roughly on par (surpassing) the features of your rt-ac68u, as would mt7622bv+mt7915e - ipq40xx or mt7621a would be slightly below your current specs, but might still meet your current requirements.
Walking on a limb, based on your nick, the https://openwrt.org/inbox/toh/askey/rt4230w_rev6 can be found starting around ~20 USD on the North American used markets, providing top specs (ipq8065+qca9984).