One thing I forgot about was the dead RAM slot issue in the 2005 PowerBook G4s. I made a thread about it awhile back and I re-read the replies I got there. From what I understand, if the DLSD has a dead RAM slot you can make up for the loss of memory by using a single 2 GB stick. This is not possible on the earlier SLSD models. I have run Leopard on 1 GB of RAM before (My Sawtooth only had 1 GB of RAM when I bought it) and I remember getting okay performance on TFF, but I would prefer running Leopard on 2 GB. So, I think I will rule out the SLSD.
Speaking of RAM, how much RAM does Snow Leopard need to be comfortable?
I've seen a lot of posts about newer Mac OSs and Windows 10, so here's my two cents on that. I personally do not plan on running Catalina. The bad reviews and lack of 32-bit app support put me off. I think Mojave would be as far as I would want to go. Another thing that puts me off of newer Mac OS in general is the System Integrity Protection because, from what I hear, it makes installing third-party apps harder. So if I wanted to, say, give High Sierra or Mojave a Leopard theme, I wouldn't be able to do it without disabling SIP. If I was going to run modern Mac OS, I would probably be looking at a used unibody (mid-2012 or earlier) instead. I did actually consider this route too, modern Mac OS, but with Leopard or Snow Leopard themeing, but the issues I mentioned made me reconsider. I think Mavericks was the last Mac OS to have the 3D dock, but I'm not sure what browser support is like or if I can still download Mavericks from Apple.
As far as Windows 10 goes, it would be pretty cool to dual boot Windows 10 and OS X, though I already have Windows 10 on a Lenovo IdeaPad. It would be a fun experiment for sure, but I don't think it is one I feel like performing at this time.