Thanks for the replies regarding my problem of needing a factory reset and the SIM card not being available.
As some mentioned you don't need the SIM card associated with your WhatsApp in your phone, they just send you a normal text message to verify once, unfortunately I left that SIM in another country, so no access to try it in roaming mode etc.
I think this Desktop version may allow me to have access to my old account, while I start a new account on WhatsApp after a factory reset (or change number).
It is a bit annoying, but WhatsApp had a beta version (developer trial sort of thing) that you could change your number and it would automatically inform and transfer your chats on all your contacts to the new number, but that is no longer available now.
This is what they really need to roll out, as it is now, I can change my number on WhatsApp before factory reset, but then none of my contacts know about this until I message them - good if you want to get away from a feme fatal, but not practical as you may miss a message you really need ... (the Desktop version might help in that, not sure... !? Maybe after number change the Desktop also stops working... !?)
Anyway, I might just wait till I get to my SIM...