Dm Oversight

So the tattoo of barrier can grant you an AC of up to 18 while you're not wearing armor, which my DM has ruled as an essentially +8 to AC, for a very rare magic item that seems strong, but we don't have a wizard or anyone with unarmored defense. He's also told us we can get 1 item of legendary quality at level 15 or 16, and we're getting close we're at level 14. I am thinking, as a Custom Lineage artificer, of getting the robes of the archmagi, my level 14 feature allows me to surpass class limitations so technically i could definitely attune to it, my base armor class therefore would be 15+Dex mod. my dex is currently 18, soon enough it'll be 20, but like at level 15, base AC starts at 15+4 so 19, with the barrier tattoo if i attune to it, my AC becomes according to my dms rule 19+8 so 27, i also have a shield with enhanced defense, so 27+4=31, and i have the shield spell always prepped cuz battlesmith is very strong. So, not including a Crit, Enemies need a +21 bonus to have a 50% Chance to hit me. At level 15? This is just not fair right? i should tell my DM to change the rule and to be careful next time, but i thought this would be kinda fun to share