How can I mount a filesystem, mapping userids?

Unlike e.g. NTFS or FAT ext4 supports Unix style permissions so the UIDs are stored in the file system and can't be changed by mount options.

The simplest thing would be to ensure both default gids exist on both machines, and your uid is in both groups.

I had to run the code below with doas, otherwise it wasn't mounted. Code: fuse-ext2 -o uid=1001 -o force /dev/ada0p3 /mnt/ada0p3.

The permissions you set on a mount point before the mount are always overwritten by the permissions of the filesystem itself in ext4 when it is mounted.

At the moment – no, as ext4 doesn't support the uid/gid options that vfat/ntfs drivers have. (Nobody seems to realize that uid= and gid= options ...

Only FAT, vFAT, exFAT support the uid , gid , umask options. You can check this by reading the list of the possible options in the mount man ...

uid=1000,gid=1000 options are used when mounting filesystem type that do not use unix scheme for users, such as windows share (know as cifs ...

You can use bindfs. It can bind the filesystem to some other mountpoint with different uid/gid. However I think I would just change the uid so it is the same ...

Explains how to use UUID under Linux to mount partitions and storage volumes using /etc/fstab file for ease of management.

A new partition will always mount with root as owner. Once mounted you can do a chown on the mount point. The device and mount point will then be reset.