old macbook pro OS update after disk repair

Im still using a macbook pro from 2011, its been a solid machine for years and my main tool for photo editing, even survived several falls from height. I havent updated the OS in years and still running Sierra 10.12.6, stupid to that have not updated the OS I know, but in the past somtimes an update has failed but nowadays i am often getting a message on web browsers etc that i need to update the browser/OS and I also cant access my icloud now on web browsers, im slightly scared to try and update it again incase it fails but recognise i badly need to update the OS to the last available version. ive used time machine to back up the mac and performed disc repair on the mac but not in recovery mode, (which I usually do before the update in the past), however my battery (not the original one) is needing replaced and on a number of occasions my machine has died, then restarted once connected to power, im just a bit worried this may have damaged the disk and slightly reluctant to try a disk repair in recovery mode incase there is a problem, but feel better knowing that I have a full time machine back up. do you think it might be magically ok and the disk repair will be ok and the mac will update to the last available version? or should i Perhaps order a new HD in preparation that this one may be damaged? its the original HD that came with the machine. and do you know the latest OS my mac will go to ? many thanks. also for context, i think there is no evidence the HD is damaged, mac is usually working normally apps like Lightroom working fine but lagging/freezing once too many tabs are open in a web browser. thanks