A few ideas and questions.
IDK what is really necessary for AAA fire control or Electronic warfare.
Would something akin to Sea Phoenix's fire control, say in this case 1-3xF-18 radar sets allow for use of something like ESSM?
If we can make a useful AAA fire control produce-able at scale then something like the old ARAPAHO project could be done, possibly using STANFLEX modules. Of course due to the dearth of U.S. ships, that would require congressional action to allow for the purchase of vessels overseas. eg:
Bulk Carriers For Sale - Vessels For Sale - Ship Sales - Advertise your commercial vessel or ship - List Your Vessel with Horizon Ship Brokers, Inc.
horizonship.com
That has to be done in one fell swoop as if the chinese see us doing this they will buy every ship on earth quite quickly.
Another, non-statutory and quite non-trivial issue with this is that most of these vessels make 14-16 kts. Also, few are in particularly good condition, after all their owners are selling them. However, depending on their crane fit, they might be productively fitted out as supply vessels or repair ships; potentially a major benefit. On the more outré side of things, as AMCs with VLS in the holds they might make Arsenal ships with other, proper, warships providing fire-control...when these slow boats from China can manage to be in range.
For new builds on the Very Low, desperation end of the High-Low spectrum I'd suggest fast oil rig supply craft which the U.S. is capable of building in some numbers. A few Stanflex modules and perhaps a van mounted sonar as was designed for the Coast Guard Medium Endurance Cutters might make them decent stop-gaps. Perhaps firing sonobouys over the side, and, at the higher end, the ASW version of the MD 50, which, I believe used to operate from the DASH hangers on Taiwanese FRAMs and might be able to fly off a very small ship.
An updated DASH drone might be looked at too.
The elephant in the room is electronics and missile production capacity. IDK how that production gets ramped up, but ESSM and RAM might be obtainable in numbers in the near term. The old ARAPAHO project and the Danish Stanflex system as well, both assumed the kit to do the upgrades and swap out modules would have their numbers built up gradually and stockpiled to be quickly fitted when the balloon went up.
There is also the very well understood possibility of USV drones, but those are likely to have even more production bottlenecks regarding their electronics than something like a Stanflex module and a fighter radar. There are probably operational complications as well that are classified.
Mine warfare is a thorny issue that likely requires some manner of sorcery to do on the quick and cheap.