Avatar of Death BBEG

So I had this idea for a campaign where Death itself is the BBEG. It has grown tired of the way adventurers so casually wield resurrection magic and is seeking to do something about it. Leading the Grim Champions from Book of Many Things, Death itself seeks to separate the material planes from the divine planes so that mortals can never again escape its design. Utilizing the Avatar of Death stat block as a baseline, how should I go about building the stat block up to make it BBEG worthy?

There are too many unknowns for answering this question.  What level is the party expected to be?  A massive campaign or something much shorter?  Are you already running a campaign and getting close to the end?  What does the party look like?  Is this for a campaign you are planning on playing or one you plan to write as an adventure (I expect it is the former, since you’ve mentioned using content outside of the Creative Commons)?

From what you have here, it kind of looks like you are only at the initial planning stage for the campaign.  Since you have picked a CR 20 monster for the boss’ followers, I expect you think the campaign will last at least a couple levels - a CR 20 is threatening to maybe level 13-14 parties (depending on composition), and most games do not start that high.  So, I feel somewhat comfortable assuming the “reading between the lines” interpretation is that you are still quite some time away from the BBEG fight

Assuming that is true, the best piece of advice I can give is that you are premature in preparing your boss’ stat blocks. You can come up with their motivations, their desires, some of the general resources they have at their disposal, etc. - everything you need to know how they will react to the party and what resources they will send to interfere with the campaign.  But the actual stat block?  That should be designed closer to when it is actually needed - that way you can tailor it to the specific party and make it feel more threatening and more epic for your individual game.

I will note, the Avatar of Death stat block is a pretty bad one to use for a basis of a boss.  It is very simple, and its main schtick is that multiples can be summoned under certain conditions.  Especially since the grim champions are more interesting monsters, you’re going to want to start with something more impressive as a base.  Vecna has a decent stat block with some unique abilities you can build from, that all feel very death coded.  There are other high level undead as well that should fit the trick.

When it does become time to build your big bad, do not forget to consider action economy.  Especially if it is a 1 v X fight, solo monsters need to be substantially buffed so they can take the same or more number of attacks (through things like lair actions, legendary actions, or powerful summons) than your party can.  This also means you should not ignore terrain and environmental effects - nothing makes a boss feel more boss-like than the idea you are fighting them in a place of power that they shaped to their advantage.