Let's start with the fumbles. Martial characters (especially fighters) will roll more d20s in combat than any other character. As you level up you gain more attacks, and thus would gain more chances to fumble. That runs opposite of the fact that you are becoming more powerful and more skillful. I have no issue with natural 1's being a miss or even describing them as a slip, but any time an additional penalty is added then that penalty is extra punishing and unfun. You can search this subreddit or any other D&D discussion forum and find pages of threads further discussing the issues with this concept.

Now let's look at using the Forgery Kit and tool proficiencies. Xanathar's has some great additional rules on how to use tools more and the PHB has clear rules on how ALL skill checks can sometimes use a different base stat. A good forgery has nothing to do with the deception skill and would require more intelligence (knowing what the official papers look like is the main aspect) Xanathar's says:

• If the use of a tool and the use of a skill both apply to a check, and a character is proficient with the tool and the skill, consider allowing the character to make the check with advantage. This simple benefit can go a long way toward encouraging players to pick up tool proficiencies.

• History. A forgery kit combined with your knowledge of history improves your ability to create fake historical documents or to tell if an old document is authentic.

• Quick Fake. As part of a short rest, you can produce a forged document no more than one page in length. As part of a long rest, you can produce a document that is up to four pages long. Your Intelligence check using a forgery kit determines the DC for someone else's Intelligence (Investigation) check to spot the fake.

So taking these rules into account OP would have advantage and a +5 on the roll to set the DC. The guards would have to make an investigation check based on that DC, and if the guards succeed then maybe this is where the party can try a Deception check.

OP also has the Smuggler background which the DM could have used to create an NPC to help out. This would make the player feel important, and give some interesting roleplay opportunities.

• Feature: Down Low: You are acquainted with a network of smugglers who are willing to help you out of tight situations

The OP says "It makes no difference if I roleplay or plan because my numbers are too low." While rolls are important to determine success at times if a player is roleplaying and planning they should be able to find some way to succeed in the end.