Hi Will, welcome to the forums.
First of all, you've made the right step when switched to RAID10 for the repository, since it has less I/O penalty and is recommended for backup methods utilizing synthetic activity.
Overall, your plan looks good and here's a handy tool
for estimating disk space required for backup repository that will probably help you in more precise planning.
120 restore points long backup chain should not pose consistency issues. However, if you have space, you can cover yourself by enabling weekly retention as well, to have some points in between. Btw, here's a good topic
on this matter.
To cut the job retention, you just need to change the number of restore points to keep in the backup job and after the next successful run it will adjust the number of backup files in accordance with the new setting. Backup copy job will copy just the latest VMs state anyway, creating just a single restore point during its first cycle, it does not copy all the restore points available on the source repository.