Core Loop

Before building big systems:

define core gameplay loop,
define smallest playable version,
define what is not in the game,
define technical boundaries.

For example:

Core Loop

travel,
manage dogs,
race,
earn money,
upgrade,
repeat.

Then resist adding:

crafting,
online multiplayer,
giant open worlds,
procedural everything,
survival realism overload.

Another Advantage You Have

Because this is not tied to:

income,
deadlines,
career pressure,

you can afford to:

restart small systems,
learn properly,
refactor,
experiment,
throw away bad ideas.

That is actually an excellent learning environment.

My Biggest Recommendation

When you finally start:

keep the first playable version extremely ugly and simple.

Like:

colored rectangles,
placeholder dogs,
programmer art,
simple UI.

You want to prove:

movement is fun,
race pacing works,
dog management feels satisfying.

Only later:

art polish,
animation,
sound,
atmosphere.

This dramatically improves your odds of finishing.