Exim - How to deliver to a user maildir beneath domain folder - smtp

Question

Hats off to anyone who has put together exim smtp server!
I have it working with dovecot authentication + dovecot imaps access, using a deovecot passwd formatted file for authentication and zero users setup on linux / no home.
Maildir delivery to a folder#local_part
But its only one domain, the delivery is always to a unique user ie bob#blah.com equates to bob.
I'd like to have a number of domains and there could be bob#blah2.com and bob#blah3.com who are both different bob.
Don't want to add linux users, or use aliases. Just want to deliver the email to a directory named after the domain\user the email was sent to.
I thought it would be as simple as creating directories #domain#local_part and then checking the recipient is authenticated and delivering to that maildir. The authentication works, but not the delivery. #domain_data is empty and #domain is "tainted" error. From what I've read I believe I have to untaint the #domain data which automatically populate #domain_data (untainted). But I've failed for a few weeks to find how.
I'm authenticating via dovecot. It has a user file in the passwd format but with sha512 password.
eg
cat /etc/dovecot/users
bob#blah.com:{SHA512-CRYPT}$6$iAlzfEx8Ft5M1::::::userdb_quota_rule=*:storage=1G
Surely the most logical thing is, once a user is authenticated, if that authentication process contains the domain, then the domain should be authenticated.
So does it need another lookup (in the router section?) to again check the same dovecot/users file to see if bob#blah.com exists, if it does, then deliver to \domain\user?
If I have to add the home dir to each entry in dovecot/users, can do that if it needs to pulled in via lookup. But I was hoping that simply matching the recipient address to an authenticated user is ok
Something like:
my_domains: driver = accept   domains = ${lookup{$local_part}{$domain}lsearch{/etc/dovecot/users}} transport = my_mailboxes
Thank you very much for any help!
EDIT:
Further info, sent email to eximm#MyDomain.uk
cat /etc/dovecot/users
exim:{SHA256-CRYPT}$5$SFHDKJqI.hDb1::::::userdb_quota_rule=*:storage=500M
eximm#MyDomain.uk:{SHA256-CRYPT}$5$SFHDKCx6v4WH.FJqI.hDb1::::::userdb_quota_rule=*:storage=500M
router
my_domains:
driver = accept
domains = lsearch{$local_part#$domain}/etc/dovecot/users
transport = my_mailboxes
transport
my_mailboxes:
driver = appendfile
file = /srv/mail/$domain_data/$local_part_data
user = exim
18:30:43.566 25510 checking domains
18:30:43.566 25510 ╭considering: lsearch{$local_part#$domain}/etc/dovecot/users
18:30:43.566 25510 ├───────text: lsearch{
18:30:43.566 25510 ├considering: $local_part#$domain}/etc/dovecot/users
18:30:43.567 25510 ├considering: #$domain}/etc/dovecot/users
18:30:43.567 25510 ├───────text: #
18:30:43.567 25510 ├considering: $domain}/etc/dovecot/users
18:30:43.567 25510 ├considering: }/etc/dovecot/users
18:30:43.567 25510 ├───────text: }/etc/dovecot/users
18:30:43.567 25510 ├──expanding: lsearch{$local_part#$domain}/etc/dovecot/users
18:30:43.567 25510 ╰─────result: lsearch{eximm#MyDomain.uk}/etc/dovecot/users
18:30:43.567 25510 ╰──(tainted)
18:30:43.567 25510 MyDomain.uk in "lsearch{eximm#MyDomain.uk}/etc/dovecot/users"? no (end of list)
18:30:43.567 25510 my_domains router skipped: domains mismatch
18:30:43.567 25510 --------> system_aliases router <--------
╰──(tainted)
MyDomain.uk in "lsearch{eximm#MyDomain.uk}/etc/dovecot/users"? no (end of list)
I've not got it right, but don't know why. My understanding was domain_data should be untainted and populated after a successful internal lookup.
Even assuming that interpretation is roughly ok, at the moment the lookup fails.
EDIT2: Should say, MyDomain is a find+replace. The actual domain is lowercase in all areas. Although I believe lsearch is case agnostic anyway.
EDIT3: I think I have a working workaround, so will update just to confirm it is working and what the solution was once confirmed.