I do not know what average rents have done in your market over the last 10 years, but in my market (San Diego) the average rent has gone up ~1K/month in the last 10 years (source RentCafe). 

What data sources do you use to pull marketing lists to mail to multifamily property owners?

I don't know where they source their data from, but it's certainly not just folks that send them business. 

The Days on Market (DOM) figure for single-family homes fell from 51 to 26;Total property sales rose 9.2 percent with 12,036 units sold;Total dollar volume increased 26.7 percent to $4.3 billion;The single-family average price increased 14.0 percent to $380,233; The single-family median price rose 15.2 percent to $310,000; Single-family homes months of inventory registered a 1.9-months supply, down from 2.6 months year-over-year and below the national inventory of 2.6 months;On a year-to-date basis, single-family homes sales are running 17.4 percent ahead of 2020’s record pace.Townhome/condominium sales jumped 27.1 percent with the average price up 9.5 percent to $237,473 and the median price up 14.3 percent to $200,000;Single-family home rentals fell 10.2 percent with the average rent up 9.6 percent to $2,153; Townhome/condominium leases rose 1.8 percent with the average rent up 6.9 percent to $1,765.Source HAR

Total new listings entered for all property types in YTD 2021 was 86,787 vs. 83,247 new listings entered for all property types in YTD 2020.Source: FMLS

Or another source of assistance?

@Astrit Bauta, regarding your final comment of the original post: why would a general partner go through all of the trouble of sourcing a deal, putting together an offering, executing on the project, etc without being compensated?  

End goal is to eventually scale my real estate portfolio and make real estate investing my main source of income.

BTW, Theres paint shortages so confirm that they have a good source even SW has been tight on some of their products.

Definitely if the cash flow is the primary source of return, there are numerous other investments that are more passive (ADU additions are far from passive) that are likely to produce better returns.