I just went through this process with Gentoo on a framework laptop. Here are some random, likely incomplete notes to make this work. It uses a swap file on an encrypted filesystem

  1.  Create a swap file.  Our rule of thumb is ram + sqrt(ram) for hibernate

      fallocate -l 72GiB swapfile
      chmod 600 swapfile
      mkswap swapfile
      swapon swapfile
      swapon --show

  2.  emerge suspend

  3.  Get the number to use with resume_offset later.  In the current case, it was 125798400

      swap-offset /swapfile
      
      emerge sys-boot/grub
      grub-install --target=x86_64-efi --efi-directory=/boot                       
                                                                                 
      vim /boot/grub/grub.cfg                                                      
                                                                                 
      timeout=5                                                                    
                                                                                 
      menuentry 'Gentoo Linux 5.18.19' {                                           
              root=hd0,1                                                           
              insmod all_video                                                     
              linux /kernel-5.18.19 root=/dev/mapper/root resume=/dev/mapper/root resume_offset=125798400
      }

  4. Fix suspend.conf

     vim /etc/suspend.conf

     resume device = /dev/mapper/root                                             
     resume offset = 125798400    

  5. Setup an initramfs

     cd /usr/src
     mkdir initramfs
     cd initramfs                                                                 
     mkdir -p bin dev etc proc sys new-root                                       
     cp -a /dev/{null,console,tty} /usr/src/initramfs/dev/                        
     cp -a /bin/busybox ./bin                                                     
     cd bin                                                                       
     for i in `./busybox --list`                                                  
     do                                                                           
         ln -s ./busybox $i                                                       
     done                                                                         
     cd ..                                                                        
     cp -a /sbin/cryptsetup ./bin                                                 
     mkdir -p ./run/cryptsetup                                                    
    lddtree -l /sbin/cryptsetup

    Copy in all of those files until the local cryptsetup works appropriately

    vim init

      #!/bin/sh                                                                        
                                                                                 
      # Define a rescue shell                                                          
      rescue_shell() {                                                                 
          echo "Error in boot process, dropping to a shell"                            
          exec /bin/sh                                                                 
      }·                                                                               
                                                                                 
      # Mount our devices.  We sleep prior to dev to hopefully finish loading.         
      mount -t proc none /proc                                                           
      mount -t sysfs none /sys                                                         
      sleep 2 && mount -t devtmpfs none /dev                                           
                                                                                 
      # Decrypt the root partition                                                     
      cryptsetup --allow-discards luksOpen /dev/nvme0n1p2 root || rescue_shell         
                                                                                 
      # Attempt to resume                                                              
      printf '%u:%u\n' $(stat -L -c '0x%t 0x%T' /dev/mapper/root) > /sys/power/resume  
                                                                                 
      # If we're not resuming, mount the new root                                      
      mount -o noatime,discard -t ext4 /dev/mapper/root /new-root                      
                                                                                 
      # Unmount (cleanup) our devices                                                  
      umount /proc                                                                     
      umount /sys                                                                      
      umount /dev                                                                      
                                                                                 
      # Boot from the unencrypted partition                                            
      exec switch_root /new-root /sbin/init
          
  6.  Suspend should be working with:

      echo shutdown > /sys/power/disk                                              
      echo disk > /sys/power/state  

      or preferably

      loginctl hibernate

Anyway, there's a lot of missing detail in there, but the idea is that there's a swapfile inside the normal encrypted root partition. For me, I've enough ram where I don't really use swap unless hibernating, so a swapfile versus a separate encrypted swap partition suffices.