A few observations:
You can simply keep extending your If statement to function as If, Else If, Else
If(
// If Branch
Checkbox1.Value = false && Checkbox2.Value = true,
If(
IsBlank(TekstZoekVeld_1.Text),
Distinct(
Lijst_Eindp,
Omschrijving
),
Filter(
Distinct(
Lijst_Eindp,
Omschrijving
),
TekstZoekVeld_1.Text in Result
)
),
// Else If Branch
Checkbox1.Value = true && Checkbox2.Value = false,
If(
IsBlank(TekstZoekVeld_1.Text),
Distinct(
Lijst_Eindp,
Omschrijving
),
Filter(
Distinct(
Lijst_Eindp,
Omschrijving
),
TekstZoekVeld_1.Text in Result
)
),
// Else
Blank()
)
I copied the first branch and just inverted the logic. You'd need to update for your purposes to include your new columns.
The DropDown usage I think is a bit backwards - at least how you've asked for it to be used relative to it being included in the function. "Whenever they type something in here, it needs to filter the amount of options inside of my dropdown (Omschrijving)" Where is this drop-down? In the function, it's used as a filter parameter explicitly. Are you trying to filter the gallery using this dropdown, or filter the drop-down based on the checkboxes being checked?