Keep an eye out for indicators of population increase or steady housing demand.The BRRRR approach may be appropriate for older properties, particularly if they require renovations.

I see lots of posts about trying to do this as a way of circumventing taxes, but nothing about doing it as a way to increase W-2 income.

When you sell your house in the future, homes with garage converted ADUs tend to attract more investors, who are not likely overbid on a property compared to those looking for their primary residences.

While this is great for the environment, typically homebuyers will not want to pay for that increase. 

I have an option to purchase at an attractive price but would rather not bring cash to the transaction.

In the US, many people use a 5-6 year rule:  if you can recoup the costs of renovations in 5 years (through increased rents) then it's worth it. 

My initial intent was to sell the property to another investor to increase my capital and invest that into other projects, but I have changed my mind and want to use it as a rental.

Will increase cash flow much more.

Shall we lease it out first with good tenant to increase the value of the building and sell it done in the road?

Unfortunately its getting harder and harder for anyone intersted in investing as an LP to vet the the sponsor groups as the tactics used to raise capital along with reprenstations made are increasingly falling in the grey area.