My ISP has /64 prefix and no RA (only DHCPv6), WAN6 can get IPv6 GUA without problem, and for LAN it's obvious that I have to relay all RA/DHCPv6 etc. However I noticed one thing, assume my GUA has aaaa:bbbb:cccc:dddd::/64 prefix, I need to explicitly adding ip6prefix aaaa:bbbb:cccc:dddd::/64 to my WAN6 section otherwise LAN & all LAN clients won't get the GUA, why is this manual operation required?
Check the output:
uci show network; ifstatus wan6
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Let me go back home and check (I guess you want to check without adding ip6prefix, right?)
However even without that line in WAN6 config my OpenWrt router is still able to get online (I can download software packages), the only issue is LAN side.
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If you get a /64 prefix which is a different /64 than the wan interface prefix, that prefix can be delegated to one LAN interface.
But now my LAN and WAN are both having same prefix (shouldn't this be normal?)
Is your OpenWrt router connected to an ISP-provided router or a modem?
Direct connect to ISP ONT.
no. In an ideal and properly configured environment each interface or layer 2 domain has its own subnet. With ipv6 this is an /64. So one subnet for the wan interface and one for lan. but...
A whole site, or home, or business should get at least a /56 (256 local network) or even a /48, 2^16 local neteorks. This delegation of a larger prefix is called prefix delegation but still the subnet on the wan interface is from a different prefix.
This is how routing is intended and should be used. Also this makes it clear where the ISP network ends and where the users network begins...
Clear?
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But my ISP (well, basically all ISP in Japan) are like this, only /64 with NO prefix delegation, NO RA.
So does that mean because of this issue, I always need to manually adding ip6prefix aaaa:bbbb:cccc:dddd::/64 to WAN6 in order for the whole setup to work?
Can't answer that. I have no experience with delay a single /64.
I can only say that this is not the way as ISP gets recommended by the RIRs.
These are the one delegation really huge networks so they can serve customer. And all clearly state that a single house hold customer should get at least a /56 delegated for internal usage. In the early days there was even the consensus that it should be a /48!
One reasoning was that even the most boring customer is expected to have at least a single private lan but also a guest network and maybe even some iot shit gear... Which already counts up to 3 distinct networks...
AFAIK with pppoe-oe there is no RA because the PPP has other methods for address assignments on link. And then dhcpv6 is intended to handle the prefix delegation.
Your router is expected to serve as dhcpv6 server as well as to announce RA.
Regarding how to make relay mode, please consult the wiki it should be there...
As I said I can only help to put it in perspective.
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A single /64 is common, if not always the case, on LTE connections. Relay mode should be used for this. The LAN devices will have IPs in the same /64. These are routable to and from the Internet without NAT.
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Yes, these two config stanzas should be enough to make it work 
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I was doing exactly the same, but, WAN6 must use ip6prefix aaaa:bbbb:cccc:dddd::/64 in order for LAN side to get the same aaaa:bbbb:cccc:dddd prefix.