How this works in say 2003, or 2019 has not changed.
I would however launch the task manager.
Exit Access and then check the task manager and be sure that Access is still not running.
Eg this:

So some of your narrative seems to suggest that a stray copy of Access is still running.
And of course in most cases, if you code is running, then you can't modify code. And ALSO be VERY but BEYOND very careful if you have a form launch with a timer control + event. Timer events and a form loaded and running will play havoc with the VBA code editor. (you want to close that form and ensure that the timer event + code is not running when you decide to edit code or make changes to such forms.
So watch out for if you launched two copies of Access.
Watch out for if/when/you are using a form with a timer.
And ALSO very much watch out if you are using any SharePoint tables. Now, it is very possible that none of the above applies to you - but one of these issues might. (and all of the above are common reasons for say you not being able to modify or change something).
So exit Access - then check the task manager - some of the issues you describe suggests that a stray copy of Access is still running (or perhaps you are using automation and creating a new running copy of Access - but that's certainly not been part of this narrative - but again a "possible" thing or issue to check.
And I assume that often and frequent while in the VBA editor, just before you try and run some code, you do a debug->compile to ensure that the VBA code does not have syntax errors.
So, there is something going on here - it just not clear what that is. But some issue or something is occurring or has occurred that is making this process painful. And the above issues and problems to look at? They are the same ones that existed say in 2003 as in 2021.
The other issue is perhaps file permissions. Today, often some folder does not have permissions or permissions have messed up the file or the folder one is using.
But a timer form running - that I would look for. Doing a debug->compile in the VBA editor would also be something I do far more frequent until whatever the issue is and found goes away. And as noted, exit Access - check the task manager to ensure that Access (or a stray copy) is not running.
Also, check your defaults for when you open a file - it should not be exclusive. Eg this:
file->options->Client settings->down to advanced settings.

the default should be shared. And same when you open a database here:

So don't use exclusive or read only when you open. So, it not at all clear what "issue" is being missed, but clearly one trip up or issue is occurring - it just a question of what.
Regards,
Albert D. Kallal (Access MVP 2003-2017)
Edmonton, Alberta Canada