If you had a 3.5% mortgage on your current home and were buying another home, transferring your low interest rate mortgage would be ideal. Unfortunately, lenders don't allow that.
When buying a home today, it would be smart to think about selling it in the future. To have a good home with unique features makes it marketable. To have attractive financing that could be assumed would add to the salability.
Consider getting a FHA or VA loan to purchase your home. The present advantages are that these loans are priced competitively and a little easier to qualify for than conventional loans. The future advantage is that FHA and VA loans are assumable at the original note rate for qualifying buyers.
There's more to sell than the home itself when you have an assumable loan. The mortgage payment could lower the cost of housing significantly. A buyer may easily be willing to pay more for the home due to the attractive financing, especially if it helps their equity grow faster.