I've tried to insert a really long random address into the SOLO settings field on my Solo Node (with the latest v2.1.1 firmware) and got a warning while typing the 62nd character:
"Warning: P2WSH and P2TR Bitcoin address are not valid for SOLO mining. Please add a different Bitcoin address"
P2WSH is a specific address type which starts with "bc1q...." but is 62 characters long (as opposed to Native Segwit "bc1q...." type address which is 42 characters long).
P2TR is a Taproot address which is 62 characters long as well.
So it looks like Native Segwit addresses ("bc1q...." which are 42 (not 62!) character long) are good for Solo mining on FutureBit Solo Node currently, and the guide contains outdated information.
Reference chart of Bitcoin address types:
https://www.unchained.com/blog/bitcoin-address-types-compared#reference-chart