So I had an answer from Magicsim Elite, and it looks really promising:
"Yes, our dual sim adapter is intended for phones with 1 SIMcard (but works also on smartphones with 2 SIMcards). Here is the page for or Magicsim adapter for your smartphone:
Buy MAGICSIM Elite - Galaxy S9 - Dual SIM Adapter for your Samsung Galaxy S9
The difference with dual sim phones is the following:
With our Magicsim Elite adapter you use 2 SIM cards in 1 phone without having to manually swap them over. With the Magicsim menu you can easily change from one number to your other number. This has the same effect as physically changing your SIM cards, but is so much easy.
You are able to receive/make calls, text messages and use your mobile data for 1 SIM card at the time. Use Whatsapp Business next to your regular Whatsapp account to use Whatsapp for 2 different numbers.
A call to your number that is not active goes directly to voicemail and SMS goes to the SMS box of the non-active number, so you won’t see that neither.
An option could be to forward your calls to your active number. Or search for an app that forwards your SMS. After receiving this SMS of the not-active number, you could swap the number and call the person back."
That looks awesome as their product will turn my 1 SIM card Samsung S9 into a dual SIM card Samsung S9, which is exactly what I’m looking for :))
My only concern so far is I’m wondering if their product will work with my phone given I don’t use a regular Android version but /e/OS instead.
So I’ve asked them also, and I will keep you updated.