With 64 GB of available space is it time for a new computer?

Can you say something about "64 GBs available space" on the HD -- and what that aspect of my question indicates

about the time to buy a new computer in addition to the observation of Catalina OS using at least 1 GB a day,

even when I am doing nothing on the machine?

Also, as an observation that may tell a more advanced tech person than myself something about the OS and its functioning: I use F4 Launchpad to view the dock on the big screen and from there on Mojave it was very quick into the app chosen; now, since Catalina upgrade, not so quick in response, most especially with MacMail app - clicking that icon and i have to go back and click it twice for it even to respond at all - as I say, particularly with MacMail app. Does that behavior give you any clues about the way the computer is functioning - and why that might be the case?

Sorry. It is a MacBook Air mid 2012, with Catalina OS 10.15.7 now installed. Also adding here in the next paragraph two other observations that can inform your opinion of if I should buy a new computer now or am I being too cautious and the Catalina mail issues are just 'Catalina' being itself?

I'd advise anyone with this year computer to stop upgrading at Mojave. One telephone Apple Support person said to stop at Mojave; should have listened; but another later said go to Big Sur. For some reason at some point there were issue with MacMail interacting with the contact app that i was told would only resolve by moving into Catalina. Now when I try to use the BCC line for recipients, the spinning ball appears between entries; as it also does during the spell check: all moving so so much slower and less smoothly than it did with Mojave.

I would not be thinking at all about any upgrading of the OS system further at this point, for sure; I agree.

But how much if any particular amount of space do I need to use for example Migration Assistant to move the data into to a new computer? That is i see now my main concern, unveiled by your prospective.

Do I understand correctly, that Migration Assistant (MA) does not need any particular number of GBs in the available category to work without any glitches?

Some YouTube advisors, like David A. Cox, report that using MA to transfer data his customers find that "glitches" happen that require them to uninstall and reinstall the OS to work around. What is your opinion of that statement; have you also heard users indicate trouble with Migration Assistant. Do Apple Specialist stores usually offer this transferring from old to new computer as a service?

Ronasara, thank you for your patience in talking me through this.

Lastly, from what i have shared about MacMail in Catalina, do you see that as a Catalina issue/challenge that you have read about before; or is it a sign that a new computer is needed? The MacAir mid2013, 4GB Memory, 64 GB available, Catalina OS behavior that i've describe in this conversation and a slow start up (after Yosemite upgrade and more so now): are these symptoms indicating this a new computer would solve those issues, or just something in the Catalina OS that i need to learn how to work-around?