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Summary of Significant Accounting Policies, Disaggregated Net Revenue (FY) (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disaggregation of Revenue [Line Items]              
Revenue recognized         $ 4,300,000    
Deferred revenue $ 7,800,000   $ 7,800,000   5,300,000    
Total revenues 50,884,000 $ 46,505,000 138,969,000 $ 117,470,000 161,790,762 $ 144,231,479 $ 101,402,007
Revenues recognized over time              
Disaggregation of Revenue [Line Items]              
Total revenues 50,884,000 44,459,000 138,969,000 112,615,000 155,362,875 143,489,929 101,402,007
Revenues recognized at a point in time              
Disaggregation of Revenue [Line Items]              
Total revenues $ 0 $ 2,046,000 $ 0 $ 4,855,000 $ 6,427,887 $ 741,550 $ 0