I looked into SNMP not long ago and found that mini-snmp wasn't all that "mini" and that it only supported 32-bit counters (not terribly useful in this era, in my opinion). Both that and snmpd require libnetsnmp which is around 370 kB plus a couple other dependencies.
I'd look into seeing if a custom-built image could bring the installed-flash space down as the ROM uses xz compression, last I checked. You might have to remove other "unneeded" packages in your build, such as LuCI, uhttpd, PPP and PPPoE kernel modules, various DHCP and DNS packages (I'm assuming that those services are supplied elsewhere on your network).
The "tiny" target is the "last gasp" target for flash-constrained devices and removes, as I recall, LuCI and a couple other things to get the image size down.