If copying blockchain to SD card isn't going to affect life significantly we're agreeing micro SDXC I/O are slow. That's put me off so I won't try it. I'll forget RPi 400 I'll go for your kit make up.
For my RPi nodes (Raspiblitz, Raspibolt, Umbrel for testing) I only use microSD cards of high quality and designed for high write endurance, specifically Samsung Endurance Pro variants. If you're tempted to get a cheap large capacity microSD card, well, I wouldn't bet how long it will last.
I'll wait to buy the best RPi 8GB with 1TB SSD. It isn't easy to buy there's stocking issues so you've done well.
Yes, RPis are unfortunately still in short supply and subject to scalpers asking ridiculous prices here and there. Never give those scalpers what they demand, at least I refuse to do that. Have a look at
https://rpilocator.com/ maybe you get lucky and can fetch a device when a shop gets them in stock. Some shops offer their customers some kind of membership offers with limited amount of devices and reserved for shop members.
I got my last RPi Compute Module 4 with 8GB RAM from such a shop's club membership (only registration needed, no extra fees or similar crap) for a totally normal price. With an appropriate mainboard such a RPi Compute Module 4 can also use NVMe SSDs which offers easier setup with TRIM support for the SSD (TRIM support is also possible for a normal RPi 4B and USB3-SATA-adapter, but this depends heavily on the adapter. Some work nicely, some are a pain in the ass.)