A standard first hour set-up can be used however, the first point I'd suggest is:

1. Only build Infantry Equipment

Trust me, you don't need anything else.
As for you National Focus, it should come as little surprise that I took:

2. Political Effort

As it gives us the fastest means of getting what we want, which is:

3. Hire a Fascist Demagogue

Now in my test run these next 2 steps occurred while the game was paused, I don't believe the order is important or that the game needed to be paused, but when it comes to in-game speed, pause is your friend.

4. Take Industry Effort
5. Prepare for Civil War

Winning unscripted civil wars in HOI4 is trivial, delete your army and have twice as many fast units as you need in training when the war breaks out. How many fast units you need is dependant on the location of hostile Vicotry Points, in our case, we need 2, so train 4 (I'd use the cavalry), make sure you don't specify a deployment location.

Now all we need to do is trigger the Civil War:

6. Expand Civil Support 3 times (assuming you haven't gone and something silly like Improve Worker Conditions)

When the 3rd Expansion of Civil Support finishes you'll be on approximately 49% stability, but not have the 50 Political Power needed to Ignite the Civil War so:

7. Save 50 Political Power and Ignite the Civil War

This thicc body of text explains winning the Civil War, skip if wanted:
Immediately deploy your 2 cavalry units adjacent the other Belgian capital, preferably on the tile also adjacent another hostile Victory Point. They'll need to have at least 1 organisation to be given an order, so I'd be running the game on 1 speed.
In my first test the AI did nothing and I was able to secure the 2 out of 3 tiles containing VPs without opposition, winning the war in a matter of hours, in the second run the AI deployed both its cavalry and secured their capital and 3rd VP before I could reach them, in said scenario the cavalry unit that secured the 2nd VP can also attack the capital VP, combined with force attack on our insane 'adaptable' general, the capital will fall and the war will be won.
Speed is key, if you screw this up by not winning within a very short period of time, the ai will reinforce and youll have to wait for Germany volenteers.

For me this results in being a Fascist Belgium around July 1936. So now begins the lead up to War with the Netherlands and Luxembourg.

9. Start down the Fascist National Focus branch

With your current resources you ought to be able to equip 24 units of 12-width infantry, but you won't have the manpower so:

10a. Save up 150 Political Power
10b. Increase Consciption
11. Start saving up 138 Political Power

While we are waiting for the political power we'll build the army, first priority is to have 24 units, so we'll order up 24 10-width infantry (I'd have them training the instant they hit the field), by the time they are all in the field, youll have 7% more manpower thanks to Fascism (OP generic tree Fascism FTW) so with a little tinkering with the main 18-width Infantry unit gives us a 20-width infantry unit without a need for Support Equipment, you ought to have enough manpower now to change your army to all 20-widths.

At some point you'll hit 138 PP, for a fast run you'll want to be on the mark with this.

12. Justify War on 3 Dutch States and the 1 Luxembourg State, should cost exactly 137 PP, leaving you with 1 PP to avoid anything breaking.

The only thing to do while you wait for war is to expand your armed forces, focus on having a full army of 24 20-width Infantry trained to level 3, any surplus military resources can go into fighters for Air Superiority, or CAS if you have researched them.

DO NOT WORRY ABOUT GUARUNTEES
Seriously WT hit 38% and the British and French didn't protect either Luxembourg or The Netherlands, not sure why, I wasn't on great terms with either of them.

Once justifications are done, pause game, join axis, declare wars but DON'T call in allies (yet).

If you have the army I described you don't need any help what so ever to win the wars, 1 division is enough to defeat luxembourg's single 4-width infantry, and the Dutch are super weak at this point in late Feburary/early-mid March 1938.
If you do find yourself stalling or losing, just call in the Germans, you ought to have enough War Participation to control the Benelux without Germany stealing something you need.

There are several possible outcomes depending on whether you joined the Axis and brought the Germans into the war or not.

1. No Axis involvement, you get to take as much territory as you like, downside is you have to Satillite the Dutch East Indies or remain at war with them. Satilliting inevitably results in Japan attacking you/them. Fine Result.

2. Major Axis Involvement, this one means you kinda screwed up and the Germans won the war for you. Priorities taking the 4 states (3 continental Dutch states and Luxembourg) before anything else. Upside of this is you can let the Germans satillite the Dutch East Indies, saving you a painful war against Japan (looking at you ♥♥♥♥ American AI). Worst Result.

3. Minor Axis Involvement, you won the war, but called the Germans in at the last moment, you get everything you want, and can let the Germans have the Dutch East Indies, this is the best result.

My fastest time is: 29 March 1938


Taking things beyond Leo Belgicus.

I'd go for leaving the Axis, joining the Allies, order 66'ing the Allies (via someone like Denmark), getting involved in China, milk China for men to invade USA with, annex almost all of USA cause you still want a US puppet with 300+ ships, join Axis again, help Germans beat Soviets, order 66 the Axis, big Profits.