Their evidence:
A NTO with date of posting of 26 Oct 2023. A Thursday.
A NTO issued pursuant to the Civil Enforcement etc. Wales Regs 2013 which provides:
Unless the contrary is proved, service of a notice or charge certificate contained in a letter sent by first class post which has been properly addressed, pre-paid and posted is to be taken to have been effected on the second working day after the day of posting.
(3) In paragraph (2), “working day” means any day except-
(a)a Saturday or a Sunday;
The purported 'Date of Service' in the NTO is Saturday 28th Oct. This cannot be.
The date of service in law was 30 Oct. which gives a latest date for payment of 27 Nov.
So you have valid grounds for a WS.
But OP, your OOT reasons need to convince, but so far my reading is that they don't.
As I understand it, you live with your parents and it is their address which you've given to DVLA - perfectly proper because you live there.
So, were you actually at home when the OfR was delivered?