Is there an easy way to estimate the number of mining nodes?
While it's possible to estimate the current hashrate from the difficulty, they are two different measures. You can find an estimate for the global hashrate on the same site where you found the difficulty. The current estimate is 162 EH/s (162,000,000 TH/s).
Using a similar approach as proposed by you, we can estimate an upper bound of  162,000,000 TH/s / 110 TH/s = 1.47M mining nodes by comparing the global hashrate to that of the fastest mining equipment.