Hi OP.
I'm glad you were able to have a straightforward pregnancy and birth of your one year old and you are clearly loving motherhood. It's not unnatural to want to share the love and wish you could do the same for you friends.
However, your absolute number one priority and responsibility in life is to your son. Also to your partner if you have one.
Being a surrogate mother is very different medically than a naturally conceived pregnancy. You probably haven't got as far as thinking if it would be using one of your own eggs or if your friend is able to supply an egg or if it would involve an egg "donor" (donors are paid £750 in the UK, more in other parts of the world).
The process is the same as any other IVF, you need to inject hormones to shut off your own menstrual cycle to prepare for the embryo transfer, regular clinic appointments to monitor bloods and transvaginal scans.
Donor egg pregnancies are known to be more complex as the body recognises the "alien" egg and will tend to reject it, similar to how the body will reject a kidney transplant without anti rejection drugs. So surrogate mothers typically have to take additional drugs in the early months to prevent rejection. There is significantly increased risk for a range of obstetric problems, including raised blood pressure, pre eclampsia, LSCS, small for gestational age baby and premature birth.
All of this means you would be likely to need lots of additional hospital appointments and might have complications requiring admission to hospital or a less than straightforward birth.
This will impact your precious early years time with your son and either your partner or other family members will need to be support backup to care for him when you are away from him.
You don't say if you have a partner, or plans to expand your family, but if things go badly you could be left in a position where you are unable to have more children of your own.
Then there is the issue of friendship.
nordicmodelnow.org/2020/01/29/i-was-an-altruistic-surrogate-and-am-now-against-all-surrogacy/
This woman offered to be a surrogate mother for her best friends - they introduced her and her husband and they were a close foursome. I have met her, she is an educated woman, married to a lawyer, who thought she had done all the research but it all went horribly wrong.
She hadn't appreciated the medical side of getting pregnant via IVF. When she started seeing red flags and feeling unsure she felt obligated to her friends as they had, by then, invested so much, both emotionally and financially, paying for an egg donor and lots of medical tests. She didn't want to let them down. She didn't appreciate the risks of a twin pregnancy although she was perturbed by the husband saying "twins would be the icing on the cake" but she was persuaded to allow two embryos to be transferred.
The pregnancy and birth were terrible, she has PTSD and after they had the babies her friends dropped her and her husband and no longer have anything to do with them.
This woman and her friends planned for a single pregnancy but the embryo split and she was pregnant with identical twins. She suffered a placental abruption and a very serious haemorrhage, although the article doesn't say, it is quite likely she needed an emergency hysterectomy to save her life. She nearly died.
These are the types of things that can go wrong, sadly there have been several deaths of surrogate mothers in USA, and one that I know of in the U.K., no doubt more in other jurisdictions around the world.
Being infertile is sad, but your friends can have children in their lives in other ways. You don't have to risk your health and the welfare of your son to provide them with a baby.
obgyn.onlinelibrary.wiley.com/doi/full/10.1111/1471-0528.14257