If you are using the last updates for the 2025 Creative Suite (aka CS2025), then Adobe needs to explain why your PDF annotations in Acrobat are being dropped by InDesign. If that is a known problem with the CS2025 products, and Adobe confirms this has been fixed in the CS2026 versions, then you have a decision to make.
Status Quo
Stay on your speedy 2017 iMac and use a different workflow to get all of your PDF annotations into InDesign. I believe that was exporting the PDF to Word from Acrobat Pro and then creating a PDF from that DOCX document.
Not ideal, but the price is right. Still, you should be planning for the future and the certainty of Intel Mac obsolescence.
New Mac and CS2026
If Adobe has confirmed the first paragraph and that CS2026 has fixed the InDesign PDF annotation issue as you have described it to them, then it will be no guarantee, but rather a leap of faith in purchasing a new Apple Silicon Mac.
The current M4 iMac stops at 24 GB RAM, with fewer CPU/GPU and RAM than you can obtain with an M4 Mac Mini Pro and a new Apple display (if you are set on 27-in) or a 32-in third-party display.
My M4 Mac Mini Pro with 14 cores 20 GPU and 64 GB RAM would run circles around your 4.2 GHz iMac. I use an LG 32-in 4K display and that extra screen real estate is gold.