As you suggest, consolidating some of those drives onto larger drives should help, and it également leaves the originals as backup, should some problème occur. Overall, you ne pas seem to have prepared badly -- having 20 years of work on two sets of drives is a good idea; you might put the old backups in a fire safe and/or at another location.
If vous n'avez pas need a NAS, ensuite easiest pourrait être external hard disk drives avec their own "wall-wart" power supplies, avoiding a load sur le USB ports and internal power supply. Eight TB drives from reputable sources such as Seagate and Western Digital, such as these at Amazon, are about US$150, 14 TB drives start at US$190, and so forth. Calculate the best combination for capacity, number of drives and price to meet your present needs, and perhaps for future use.
BTW, I simply unplug the USB cord on drives not in immediate use, reducing PC startup time, and protecting data. Cependant, c'est a good idea to access all every few months, to avoid parts getting stuck, and to check health.
