So much to unpack in your post op, for me at least, but I'll have a try.
My parents were both only children. Mother was adored and loved being an only. Father came to the UK in 1939 from Germany and his family were all, except for one aunt, lost to the Holocaust.
Shall we say their marriage was somewhat tense and it was made clear, by mother, that she had only got married because she was pg and never wanted children. They separated when I was 12. I yearned for brothers and sisters and felt very lonely but that may have been more down to feelung unwanted (not by father). Also there were far more comments then about "oh an only, you must be spoilt". To this day MIL a former teacher makes snidey comments about only children she taught but over 30 years acquaintance I think that's more to do with her than anything else.
Comparing mother and I, I think a lot comes down to the reason one is an only. She was wanted and deeply loved and my gps wanted more. I was not and felt it and it was compounded by snide comments from beyond the family. And yes I had everything: pony, fab home, clothes and every material thing a child could want.
DS is one of three and has two sisters but they both live abroad (other side of world) and went as soon as they could. They rarely visit and one in particular is more selfish than any only I have ever met. Neither will be around to help with their mother and neither put themselves out when their father died. DH and I are in agreement that he will look after his mother and I shall look after mine. They are both 83.
One thing I always knew was that I wanted to be a mummy, from when I was a tiny girl. For all the reasons above I wanted three or four children. Just before DH and I got engaged I started to feel unwell and was diagnosed with severe graves - my legs ballooned and it was suspected my heart was under strain. It wasn't. The overactive thyroid at the time was brought under control with carbimazole and I elected to have a sub total thyroidectomy rather than radioactive iodine, both of which would render me hypothyroid but this was more easily controlled and the best option when planning a family. Interestingly back then, 1989, the view was that once an overactive thyroid was controlled there was no impact on pregnancy. My endocrinologist with this view was none less than the eminent Sir Richard Bayliss. To be entirely fair I can honestly say I have never had unorotected sex and not got pg. However:
Pg 1: miscarried 6 weeks
Pg 2: miscarried 17 weeks - failure of placenta
Pg 3: DS 1 at 36/3 - cord round neck, and thyroid had gone under but midwives were sloppy with blood tests (link now known re early deliver)
Pg4: MMC 12 weeks
Pg5: DS2 diagnosed with a severe congenital heart defect at 20 week scan (not thyroid related) born at 27 weeks and did not survive.
Pg6: MC 5 to 6 weeks
Pg7: dd, 41/4 pink, fat and screaming after a v easy labour.
We stopped at 2. It took me 1993 until 1998 to produce two healthy babies. DD was born just before my 39th birthday. I couldn't face losing another. Not trying for a third is my one regret in life. It is good the link with thyroid disease is better recognised and monitored now and I don't think thyroid disease alone should put you off. Perhaps I just ploughed on in blissful ignorance but I was determined ds wouldn't be an only like me.
I am sorry as I suspect that has turned into an essay.
Good luck whatever you decide 