[RPG] How to the character start a thieves’ guild without being disruptive to the rest of the group

Background: D&D 5e with a steam punk flair.

My protagonist is a changeling rogue (can assume the role of other people) with a small bit of magic (Arcane Trickster). My deity has requested that I seed a thieves guild to act as a network of spies that will slowly grow as the protagonist progresses in their adventure.

In this question, I would like to focus on how I could go about doing this without being disruptive to the main storyline, or others at the table. I want to do this for fun, but not detract from the fun others are having.

On Page 186-187 the PHB discusses "Between Adventures":

Between trips to dungeons and battles against ancient evils,
adventurers need time to rest, recuperate, and prepare for their next
adventure. Many adventurers also use this time to perform other tasks,
such as crafting arms and armor, performing research, or spending
their hard-earned gold.

In some cases, the passage of time is something that occurs with
little fanfare or description. When starting a new adventure, the DM
might simply declare that a certain amount of time has passed and
allow you to describe in general terms what your character has been
doing. At other times, the DM might want to keep track of just how
much time is passing as events beyond your perception stay in motion.

While this doesn't explicitly talk about seeding a underworld super cult that you will one day use to take over the world, I think this would be the appropriate place for this activity so as not to be disruptive. What I don't know is how much time/money investment should go to this.

From "Building a Stronghold" (DMG, p. 128) – thanks @John:

A character can spend time between adventures building a stronghold. Before work can begin, the character must acquire a plot of land. If the estate lies within a kingdom or similar domain, the character will need a royal charter (a legal document granting permission to oversee the estate in the name of the crown), a land grant (a legal document bequeathing custody of the land to the character for as long as he or she remains loyal to the crown), or a deed (a legal document that serves as proof of ownership). Land can also be acquired by inheritance or other means. Royal charters and land grants are usually given by the crown as a reward for faithful service, although they can also be bought. Deeds can be bought or inherited. A small estate might sell for as little as 100 gp or as much as 1,000 gp. A large estate might cost 5,000 gp or more, if it can be bought at all.

Once the estate is secured, a character needs access to building materials and laborers. The Building a Stronghold table shows the cost of building the stronghold (including materials and labor) and the amount of time it takes, provided that the character is using downtime to oversee construction. Work can continue while the character is away, but each day the character is away adds 3 days to the construction time.

\$\begin{array}{|l|r|r|}
\hline
\textbf{Stronghold} & \textbf{Cost} & \textbf{Time} \\
\hline
\text{Abbey} & 50,000\,\text{gp} & 400\,\text{days} \\
\text{Guildhall, town or city} & 5,000\,\text{gp} & 60\,\text{days} \\
\text{Keep or small castle} & 50,000\,\text{gp} & 400\,\text{days} \\
\text{Noble estate with manor} & 25,000\,\text{gp} & 150\,\text{days} \\
\text{Outpost or fort} & 15,000\,\text{gp} & 100\,\text{days} \\
\text{Palace or large castle} & 500,000\,\text{gp} & 1,200\,\text{days} \\
\text{Temple} & 50,000\,\text{gp} & 400\,\text{days} \\
\text{Tower, fortified} & 15,000\,\text{gp} & 100\,\text{days} \\
\text{Trading post} & 5,000\,\text{gp} & 60\,\text{days} \\
\hline
\end{array}
\$

How can I do this within the rules of D&D 5e without being disruptive to the existing game?