So what do you advise? An HDD for cold storage and an SSD for OS, programs and files in use?
SSD for all purposes unless budget forces you to HDD.
OS and all applications on SSD for sure.
"Personal data" ideally on an SSD, either the same one as OSS/apps or on a separate SSD. Personal data on an HDD only if you cannot afford SSD.
In any case, the OS/applications and personal data should be backed up to some other drive on a regular basis.
So you might have:
Drive 1: OS/apps and all data; an SSD; as large as necessary, with room to grow.
Drive 2: backup of everything on Drive 1; ideally SSD. HDD if you can't afford SSD.
or
Drive 1: OS/apps; an SSD; probably fairly small, possibly 500 GB
Drive 2: all data; ideally an SSD; possibly an HDD if you can't afford SSD
Drive 3: backup of Drive 1 and Drive 2; quite possibly HDD if you cannot afford SSD.
You decide how much capacity you need on the drives. You are the only person who has that knowledge.
Ordinary SATA 2.5 inch SSDs are fine.
M.2 2280 or NVMe drives are optional and may benchmark faster, but you might not notice the difference from ordinary 2.5 inch SATA drives.