@David Schmiediche, after COVID tenant demand was CRAZY. So, I don't think anyone had trouble finding tenants. It has certainly abated. Here are a few comments on what you mentioned:
1. 600+ credit score. That is not unreasonable. However, in my market I get a lot of renters under 30 where they may have little credit history and only small negative things impacting their score. If I have a 25 y o with a 575 score who has very little credit history and a $200 cell phone bill in collections from when they switched carriers, I will often accept them. I will usually ask them questions about the items on their report and make a decision depending on the level or responsibility they show.
So, for me a good score is an easy YES, but a bad score just means I need to look more carefully and ask follow-up questions.
2. Income 3x rent. That is a very good metric to use but commonly landlords make the mistake of using NET income when it should be GROSS (before tax) income. This is the same way a lender would qualify borrowers for a mortgage.
3. In addition to the Income 3x rent metric, I go a step further! Sometimes people have enough income, BUT they have an outsized amount of debt! So, I additionally require that:
Monthly Rent + Monthly Debt Payments <= Monthly Gross Income x 45%
Again, this is similar to how lenders underwrite borrowers for mortgages.
4. You mention co-signers. I personally RARELY use them. I will only consider them if the co-signer is "collectible". If I have to sue or evict the tenant, a co-signer is of no use unless I can reliably FORCE them to pay. Usually that means they need to own real estate because a unpaid judgement can be made into a lien on their property and that makes eventual payment MUCH more likely.
5. Have you looked at your value proposition for tenants? In my area the average rental has improved noticeably in the last 5 years. So, the competition is higher.
When I have a B class rental, I will look at other B class rentals and determine what I think are "much haves" for that type of rental and then I will determine a list of features that are "nice to haves". I will try to add whichever "nice to haves" are cheapest and easiest to my rental. That might be providing a washer/dryer or allowing pets as those things are far from a given in my market. This is one way I set myself apart from the competition.
6. Another way to set yourself apart is pricing! MANY MANY landlords focus on the monthly rent solely as their measure of success. I don't. Vacancy/Turnover is VERY costly, so I price my rentals to meet or exceed my budgeted Vacancy rate.
I budget 5% of rental income for Vacancy. Since I typically take 4-6 weeks to turn a property, that means a turnover every ~2 years. So, if I price my properties to exceed that on average, I'm actually making MORE money overall than if I priced the rent higher and had tenants staying less than 2 years on average.