I read your post with interest. It is a topic that I often turn over in my mind. What I have ultimately decided is that each couple must prayerfully choose their own way, and it is not for us to judge the decisions of others. We are not privy to the private challenges each person, marriage, and family face. As long as we counsel with the Lord and obey Him, we will not be sorry.
Although you had many quotes from David O. McKay that were spoken in Conference, the bulk of the quotes are old, and not spoken by a prophet of God. I did my own search online and couldn't find any quotes from more recent prophets. Not that President McKay's counsel isn't still in effect - we know we should obey past Prophets - just that 70 years later times are different and sometimes counsel does change.
From the Church website is this information:
Official Church Policy regarding birth control:
Children are one of the greatest blessings in life, and their birth into loving and nurturing families is central to God’s purposes for humanity. When husband and wife are physically able, they have the privilege and responsibility to bring children into the world and to nurture them. The decision of how many children to have and when to have them is a private matter for the husband and wife.
God has a plan for the happiness of all who live on the earth, and the birth of children in loving families is central to His plan. The first commandment He gave to Adam and Eve was to “be fruitful, and multiply, and replenish the earth” (Genesis 1:28). The scriptures declare, “Children are a heritage of the Lord” (Psalm 127:3). Those who are physically able have the blessing, joy, and obligation to bear children and to raise a family. This blessing should not be postponed for selfish reasons.
Sexual relations within marriage are not only for the purpose of procreation, but also a means of expressing love and strengthening emotional and spiritual ties between husband and wife.
Husband and wife are encouraged to pray and counsel together as they plan their families. Issues to consider include the physical and mental health of the mother and father and their capacity to provide the basic necessities of life for their children.Decisions about birth control and the consequences of those decisions rest solely with each married couple. Elective abortion as a method of birth control, however, is contrary to the commandments of God.
(Encyclopedia of Mormonism - which I know is not doctrine, but they do quote the General Handbook of Instructions which is at my husband's office at Church so I couldn't get to it). The general handbook of instructions for Church leaders has the following instructions concerning birth control: "Husbands must be considerate of their wives, who have a great responsibility not only for bearing children but also for caring for them through childhood…. Married couples should seek inspiration from the Lord in meeting their marital challenges and rearing their children according to the teachings of the gospel" (General Handbook, 11-4).
Interpretation of these general instructions is left to the agency of Church members. One of the basic teachings of the Church, however, is that spirit children of God come to earth to obtain a physical body, to grow, and to be tested. In that process, adults should marry and provide temporal bodies for those spirit children. For Latter-day Saints, it is a blessing, a joy, and also an obligation to bear children and to raise a family.
One of the cornerstones of the gospel is agency or choice. Latter-day Saints believe that everyone will be held responsible for the choices they make. Many decisions involve the application of principles where precise instructions are not given in the General Handbook of Instructions or in the scriptures. The exercise of individual agency is therefore required, and Latter-day Saints believe that personal growth results from weighing the alternatives, studying matters carefully, counseling with appropriate Church leaders, and then seeking inspiration from the Lord before making a decision.
Church members are taught to study the question of family planning, including such important aspects as the physical and mental health of the mother and father and their capacity to provide the basic necessities of life. If, for personal reasons, a couple prayerfully decides that having another child immediately is unwise, birth control may be appropriate. Abstinence, of course, is a form of contraception. Like any other method, however, it has its side effects, some of which may be harmful to the marriage relationship.
Prophets past and present have never stipulated that bearing children was the sole function of the marriage relationship. They have taught that physical intimacy is a strong force in expressing and strengthening the love bond in marriage, enhancing and reinforcing marital unity.
Decisions regarding the number and spacing of children are to be made by husband and wife together, in righteousness, and through empathetic communication, and with prayer for the Lord's inspiration. Latter-day Saints believe that persons are accountable not only for what they do but for why they do it. Thus, regarding family size and attendant questions, members should desire to multiply and replenish the earth as the Lord has commanded. In that process, God intends that his children use the agency that he has given them in charting a wise course for themselves and their families.
When I was a teenager, or even when my family was young, I was sure I wanted a huge family and that I could handle it. Now that I'm actually in the throes of a large family, I'm learning to recognize some of my own weaknesses and limitations that I wasn't aware I had. Some of them make me stop and think before piling child upon child into our family. I need to be able to take good care of the ones I have. The Lord did give us Agency in this matter - just as he does in all matters. There really isn't any one size fits all answer. I'm glad that you seem to have found your personal answer. I haven't found mind. I know that at this point there's a good chance I won't have any more children, but the reasons for that are mostly due to my mental and emotional abilities, and nothing physical or financial. If the Lord wants to bless me with more children, I will be happy to have them. Right now I think he wants me to rest and the final decision will come later.
As a side note, if you look at the 15 Apostles, the number of children they have had is as follows, from largest to smallest:
Maybe at least half of them had fertility issues, or maybe they limited their family. I don't know. I do know that it is obviously a personal decision for each couple to make with study and prayer.