tip/trick/recomendations requested

Concerning your weight loss, you have simply reached a plateau. You now have 3 options:
1) Reduce your caloric intake by eating less
2) Increase your intensity per given time frame
3) Increase your ride time

You mentioned "to vary the pace with time", which is called intervals. Research cycling intervals and you will learn that they can help increase your cycling fitness level while not particularly increasing your time spent on the bike.

The Trek 7200 is an adequate bike, but so are many others. Many variables should contribute to your decision: $$, riding surface, riding position, etc. Fit and "how it feels to you" should be your biggest factors.

You are on the right track. If you were losing weight before (1 lb per week is a good goal) then keep on doing what you were doing. Something may have changed in your diet or ride times if you have stopped losing weight.

As you lose weight, you'll need to go a bit faster to burn the same amount of calories. Your perceived exertion should be similar, so keep a long slow burn going. Try to ride over 1 hour in each session. Drink a lot of water. Take one day off per week.

Afer many years of cycling,mainly commuting and three/four/five hour rides of
a weekend,my advice wd be a lot of a little is better than trying long rides
on big gears and sickening yourself of riding a bike.The only reason I ever had
for increasing miles and improving fitness was to ride longer distances in comfort and to be able to climb some of the hills-it's more exhausting walking
a bike upill than riding it,however slowly.
Depends on yr motive.If you simply want to enjoy being out on a bike,particularly on a mountain bike away from traffic,then just enjoy.If yr out to
join the death or glory boys yu wd need to step up to another level : my opinion of that brigade is 'who cares who dares '?
And don't believe you will burn off all the calories whatever yu eat.For my youth I was built like a whippet and believed all the statements that I could eat anything.The later development of the mature spare tyre soon disabused me of that delusion.
Good luck with whatever yu want to achieve on the bike but most of all ENJOY.