odhcpc is provided by the upstream distro and as far as I know not being patched in the TOS build conf.
It seems to support option 5 (CER ID) but not option 61  which to my understanding is the client submitting to the upstream server its client-id, comprised of DUID + IAID.
I am not sure whether OpenWRT implements the generation of a DUID for any of the router nodes at all, not sure whether it even makes sense in router context to generate a DUID at all for submission to the upstream server.
Most ISP would utilise a IPv6 prefix delegation with PPPoE and/or VLAN as authentication mechanism, but Sky apparently has gone a different route with that option 61.
The question is whether Sky requires the subscriber’s router to submit a client-id via DHCPv6 for general connectivity or whether it is just required for end nodes, e.g. TV, streaming boxes, etc to be able to access certain ISP services.
Maybe this  could be of assistance on the subject.
If the ISP’s box in full bridged mode performs well and does not cause any issue why to invest in another modem? If however, aside from the mentioned modem and depending on the subscriber line, there might be SFP modems to consider, saving space and an extra power supply/socket.
That is sometimes an issue with ISP boxes not ceding full control to the subscriber and sometimes even having a remote backchannel (often dubbed support) in the box.
Just in case the box does not provide full bridge mode but only some sort of half bridge mode be aware of potential duplications with DHCP, DNS, Firewall that could easily cause grievance with downstream devices.
I just picked that one as it looked promising, however a search for sky dhcp 61 in that forum turned up also other hits.