If he is a long term, no problem tenant, I would gradually increase his rent over time and by that i mean years.

any advice for someone who wants to invest gradually, without risking a large figure from the beginning?

This is how townhouses get built,  for example a 50x100 lot is divided into four 25x50 postage stamp lots.Also,   some SFR areas adjacent to 'urban village' designated areas and arterials were upzoned further to 'lowrise'  (low to mid density multifamily) Since the only long term way to handle demand for more housing is to build more housing,  and because (left coaster here) I'd prefer more density to more sprawl into what rural areas we have left I generally support a gradual upzoning process.   

Let's say my objective is to supplement some of my wife's income gradually to eventually have her stay at home.

If you do let them tie up the property you should have a gradual pain point.For example 90 days and 50,000 earnest money of which thereafter 20,000 becomes non-refundable.

Rates have been increasing gradually over the past several months.

Sorry, I can be a dafty but if said a number of times in different ways and questions asked differently,  and once I am actually doing the paperwork it will gradually sink in.

You will gradually start to learn what is needed in a contract.

A few people will start to move in for constuction right away, and then gradually until opening. 

I foresee a slow gradual build on the west side of the island.