The difficulty level of any trek is decided by looking into different factors together.
Altitude
Altitude is one of the biggest factors that decides the trek difficulty. High-altitude means low-oxygen level. When you start ascending, the oxygen level decreases. Less oxygen can make you feel tired or dizzy.
Distance & Duration
Walking 5 km on plain areas and walking 5 km on the mountains is very different. Sometimes, ascending even 3-4 km on the Himalayan can be very tiring. Long trekking hours for multiple days can be very exhausting.
Terrain
The terrain of the trek can also impact the difficulty level. Walking in forest areas and meadows is easy. Whereas, trails like snow fields, glaciers, rocky ascents, river crossing, etc can be very challenging.
Weather Condition
Weather conditions can be a big concern on an Himalayan trek. Correct weather means better trails and good views. Meanwhile, unfavorable weather conditions like sudden rain, snowstorm, etc means slippery trails and poor visibility.
Your load
Every kilogram of your backpack means extra effort while trekking. Trekking with a heavy backpack on Himalayan trails can be extremely difficult.
Look for all the factors that concern the trek difficulty and then find the best trek for yourself.