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Most of the other official boards have 64kB RAM, 512kB Flash, so maybe this should be set as the target.
Then you are looking at <40kB RAM and 40-100kB Flash free on those.
Thread support should only be a firmware change
Adding Thread running concurrently with BLE is even more overhead than adding just 6LoWPAN over BLE (and both should provide more or less the same thing?). And in both cases you also need some ipv6 stack (like lwIP) on top of that to send/receive UDP, all this takes some additional RAM and Flash. And of course a lot of developer time to get this working. I think you may be underestimating complexity of all this 'firmware thing' a bit :-)
Espruino on ESP32 + wifi looks much easier for starting this now. Or for official boards start here https://www.espruino.com/Internet
BLE / IP: Yes, it is based mostly based on IP except for the initial provisioning where point-to-point connection is done over BLE or WiFi direct.
A bridge can be used to bring a BLE device on an IP network, but Thread protocol is supposed to solve this by allowing the device to directly join a Thread network.
So maybe bringing Matter to Espruino will only make sense after Thread is supported? (note: Thread support should only be a firmware change, since it uses the same hardware as BLE / WiFi).
RAM/Flash requirements: I think Bangle.js 2 has the correct specs
Most of the other official boards have 64kB RAM, 512kB Flash, so maybe this should be set as the target.