To Hit, it would be 5(20STR) +6 (lvl20) +3 (mgc wpn), +4 (max bless result) = +18 to hit. I think they had someone mention something else that might give an additional +2 for a total of 20, but can't recall what it was. I think if you used a ranged weapon you could have an additional +2 in fighting styles for +20 to +22
18 (Plate) +3 (mgc armr) +2 (shield) +3 (mgc shld) +1 (fighing style defense) +2 (Haste) +1 (ring of protection) = 30
If you actually can caste Shield on you and have someone else caste haste and concentrate you get after you have a reaction, for one round
+5 AC = 35 AC for one round.
So with magic items and spells this is no big deal. Without magic items and spells and a 100% defensive position the highest you are going to reach would be 18(plate) +2 (shield) = 20. The magic items you listed are essentially the best available and whenever another player uses a spell on you they are losing that option for themselves (checks and balances). Likewise, choosing to go 100% defensive means that you can't do any damage (again checks and balances). This is a non-issue.
The same is true of your hit calculation. Without the very best magic items and without spells cast on you, +5 (20 str) +6 (proficiency bonus) is +11 to hit. Again, this is a non-issue.