Pitch more yeast onto already fermented batch?

I brewed a beer a few weeks ago with a very high OG of 1.09. It was a German brown ale which we ended up pitching WLP036 Dusseldorf Alt yeast onto. It was straight from the vial, no starter made. The WLP036 has an alcohol tolerance of "medium" and produces a "slightly sweet alt beer".

When the fermentation was complete, it resulted in a FG of 1.03 and a very, very sweet and heavy beer. I tasted it when I took a gravity reading as tranferring to secondary. So primary 14 days, in secondary now 6 days.

I am almost positive there was not enough yeast when pitched and the strain could not handle the high OG.

My questions are: can you pitch additional yeast onto this beer and get more fermentation, or is this a terrible idea? Is the sweetness just a result of the type of yeast i used or is there fermentable sugars remaining in the beer due to under-fermentation?

Should i just chalk it up to experience and move on, let the beer age more, or some other method, etc? It just tastes almost like syrup (but with alcohol of 8%!)

I also washed the yeast from this batch and have it chilled in the fridge (i dont really know why cause its my least favorite batch to date). Should i consider making a starter and pithcing the same yeast back onto this beer? I would def prefer to have more alcohol and less sweetness!

Semi-new brewer here! Thanks guys

Normally I tend to leave things as they are and not tinker, because tinkering makes it worse.

However in your case here I would pitch some new yeast. I'd just go with a neutral dry yeast like Safale 05 and rehydrate it. If it tastes like syrup you have nothing to lose but a few bucks on a packet of yeast.