How to effectively organize RAID1 on two HDDs of different sizes?

Question

NAS

Available:
- NAS, QNAP D2,
- HDD1 (3TB)+HDD2(8TB).
Please tell me how to organize the following data storage scheme more correctly:
Space 1 = HDD1(3TB) + part of the space (3 out of 8TB) with HDD2 with full mirroring by RAID1 type
Space 2 = HDD2 on the remaining volume (5 out of 8TB) - normal space, file dumping
Scheme for clarity by embedding
Thanks in advance