OK managed to fix my issue.. it was a bit of a roller coaster; but got some good help from apple support - here's what happened
1) After trying to migrate a recent time machine backup from my mac pro tower to my new MBA, my MBA started dropping wifi (about every 10 seconds).
2) I suspected this was due to the time machine migration, as I didn't notice any problems when I first un-boxed the MBA. Checked all obvious network settings, nothing obvious.
3) To confirm this was a software / config problem (from time machine migration) I rebooted into recover mode (long hold power button when booting up; then choose 'options').
4) From here you can run safari from within recovery mode, and try tools like fast.com or whatever your favourite network speed testing website is.. Internet was working well, 550 Mbps from recovery mode; could barely connect to a website from within OS X. This proved in my situation that it was a software problem, not a hardware problem; hence the following steps.
5) I choose to 're-install mac os' following apple silicon specific instructions here:
https://support.apple.com/en-au/guide/mac-help/mchl82829c17/mac#:~:text=In the Recovery app window,Reinstall macOS on your computer.
6) This didn't fix my wifi problem so I kicked it up a notch and performed 'erase and re-install mac os' from the same guide from step 5
7) This created another problem, now I couldn't get past the 'create account screen' e.g. when you've finished installing, and are setting up your account for the first time; It wouldn't let me create an account on my computer no matter what; no matter whether I skipped iCloud, nor whether I created a 'test account' using a completely different name.. Nada; couldn't get into OS X
8) Repeated step 6 again; but noticed in Disk utility that a separate 'data' partition had been created; e.g. now there was two when originally there was only 1. So I deleted both of these (deleted not erased) and then erased Macintosh HD again & repeated step 6
9) Still no dice. Got online with apple support. Ultimately it seems like some sort of security bug (feature?) that makes it really hard to do a real clean re-install of OS X. You'll notice during step 6 that you can't erase & re-install unless you sign into recovery activation with your iCloud id again (e.g. to discourage people from stealing these computers).. So really it's not a clean recovery at all because your account details are still in there somewhere (e.g. maybe the t2 chip).
10) Next, boot into recovery again. This time open terminal and type (without the quotes) 'resetpassword' and press return. A password recovery window will appear. Oddly, leave that window there & in the top left of the screen under the 'Recovery Assistant' menu, choose 'erase mac' and follow the steps to erase the mac
11) Oddly also, after the computer reboots & you re-enter your wifi details & such; the next step is to repeat step 10; that is choose terminal; type 'resetpassword' again and again with the reset password window open choose 'erase mac' from the 'recovery assistant' menu
12) Your mac should now be 'activated' without you having to enter in your icloud login details.. This means that your mac is ACTUALLY now wiped clean. I was told to re-install big sur using the menu, except there was no volume available to re-install on (e.g. couldn't choose macintosh hd during the setup because it wasn't there).
13) Then I was shown this other apple silicon instruction page:
https://support.apple.com/kb/HT211983
14) we tried the terminal thing (e.g. pasting in all of the commands in step 3) of the terminal instructions. That didn't work because there was no folder /Volumes/Untitled
15) So we went back to disk utility and 'erased' the internal drive to create a new Macintosh HD. This failed on first atempt, but created a propper disk heirarcy (e.g. device name->disk name) under 'internal). So selecting the sub disk created under the device that was just created, tried 'erasing' this again. Which worked.
16) Exited back to recovery utilities, and 'reinstalled big sur' again.
17) This time everything worked.. Was able to set up a new account again & log into big sur, wifi is fast and solid. Not going to try my luck with time machine again; going to manually re-install things one at a time.. That way when/if it breaks again I'll know what the cause was.