Hi. I'm sorry you are dealing with this. You are right to be rethinking a "full hysterectomy" since the uterus and ovaries have lifelong functions. (Ovary removal is actually a separate surgery called oophorectomy or ovariectomy. Hysterectomy is removal of the uterus only.)
You shouldn't need any organs removed and, at this point, there's no indication of surgery being needed.
A 3cm cyst is small and most cysts resolve on their own. And even when they don't, the large majority are benign. Periodic ultrasounds should be the standard recommendation at this point. If the cyst continues to grow, ~7cm seems to be the point at which they want to do surgery due to the risk of ovarian torsion. If the cyst continues to grow, it is best to have just the cyst removed (cystectomy). Removal of even one ovary has been shown to be harmful. So you would want to find a surgeon who has good ovarian cystectomy skills.
As far as the 18mm endometrium: At what point in your cycle was the ultrasound done? This isn't much thicker than normal when it is close to your period starting. Also, if you are perimenopausal (hormonal changes and cycle irregularities leading up to menopause), the lining typically gets a little thicker than normal. Additionally, if you have small fibroids or polyps those can be difficult to differentiate from the lining itself making it appear thicker than it is.
What did your doctor say about all this? If he recommended surgery, that obviously seems overkill. An endometrial biopsy may be warranted but rechecking a day or two after your next period would seem to be an option.