I beat Shadow of the Erdtree without any summons or spirit ashes and it was best decision I've ever made in gaming.

TLDR at the bottom.

Backstory: Elden Ring was the first Soulslike game I have ever played. I was god-awful at first. Eventually got better and better. Even with the mimic I would struggle. Melania was not even in the world of being possible in my eyes without my mimic. Here is the straw the broke the camels back for me. On my very first run through ever, I finally get to Melania. I have seen the streams, the difficulty, and this was at the time Let me solo her was at it's peal popularity. I was getting dunked on over and over and before I went into the mist, I saw a summon sign. "Let me solo her". While this wasn't the original guy, just a copy cat because I play on Xbox, I thought "There is no way, he can actually pull this off." I summon him, he pots up, seppukus with his Godskin Peelers and off we go. I just saw and watched. To my actual disbelief he gets completely kills her without one hit. When she was killed, I contributed nothing and I felt and actual pit in my stomach. All my work this entire game was not leading up to Radagon or the Elden Beast, but Her. I wanted to say I killed her but in my folly I got caught up in the hype. I eventually went back NG and deleveled my character and beat her with the same stats, etc.

Going into this DLC I told all my friends that would not happen again under any circumstance. No summons, no mimic tear, no NPC summons. Nothing at all. And is has been the most rewarding experience in my gaming career. My friends felled bosses so much faster than me and got through so much more content than at a quicker pace because of the mimic tear but felling Messmer, Rellana, Dancing Lion, Midra, and even Radahn has been absolutely nutty. The dopamine release is like none other. This is the definitive Souls experience and I have been converted. I can't wait to go back and use summons on NG+ DLC with all the new Ashes but for my first go around, it has been like no other. I wish I did this on my first play through on base game.

TLDR: No summons at boss fights has been extremely rewarding.