I do think the letter you have right now is rude. And this is coming from someone who has successfully gotten our family to buy a few higher quality wooden gifts instead of a bunch of plastic ones, and i did it by sending out emails. I dont always think its necessary to call...im actually a lot better at getting my message across sincerely through writing, so its better for me, but most people are the opposite.
If you are still planning on communicating this way instead of in person, i think you need to choose one angle and stick with it. Either you have valid reasons for opposing certain toys, or you are just shooting out some suggestions (the wishlist).
1. If you have no space, you can let everyone know that and kindly suggest that they try their best to keep toys small, outdoor and/or a certain amount so that you child is able to keep everything and have room to enjoy it. No other restrictions.
2. If you are more concerned with the plastic, you can express your concerns with health risks, the production process, the marketing for children, heirloom quality, etc. Whatever your reason for prefering wooden toys. One thing i dont think youll ever be able to do, however, is simply say that you prefer them because of how they look, etc. Theyre for your son, not you....so unless there is an ethical reason for opposing certain products, i dont think asking outright for these restrictions will ever be received gracefully. I think the only way to navigate a parents preference with no other cause would be a wishlist, and that will take a long time before they get the hint.
3. Educational philosophy. You say that you want open ended toys, so you can give a short and sweet explanation of the educational philosophy youre following (focusing on imagination, no mindless button pressing, etc.) And then give some links to toys that follow this philosophy to give them an idea of what to keep an eye out for.
The last piece of advice I have is to not send this out this year. It is far too close to the holidays and you WILL make some people worry about the gifts theyve already gotten. Youll seem ungrateful before you even have a chance to receive them, and you probably will be disappointed because they already got your child things that are explicitly against your wishes. Just dont do it. Wait until next year and send it out about 3 months early. Throughout the year, drop hints...talk about the things you want your child to learn with, great toys youre looking at getting, the amazing quality of wooden toys that have held up compared to plastic counterparts. Keep casual conversation about your parenting philosophies, including the stuff in your home. Then drop the email.