If you're considering alternatives to Ahrefs - also check out https://www.seowl.co/.
It has a free trial. Also - indie developed. nice to support the little guys from time to time.
(excuse me while I get boo-ed off of this subreddit)
Obviously this would be a cost compared to GSC but you should use a dedicated rank tracker for that instead of SEMRush/Ahrefs. SEOwl would do the job although for that many keywords you would need an agency account.
I would say SEOwl because it helps me get actionable information AND monitor progress at the same time. What I mean is that I can use it as a reporting/monitoring tools (like many others) but I can also use it to give me insights on the why a SERP change occured (for example : did my competitor changed its meta-description?) using their serp diff tool