In Chinese medicine a strong tight anus plays an important role in relation to a man's erection.
Are there some who have experienced problems with erection after LIS or botox? (short or long term problems)

i think you are confusing pelvic floor muscles with the anal sphincter. erection strength can be improved by strengthening you pelvic floor, not that actual sphincter itself.
I have had both Botox and LIS and it has not effected erection at all, i would not worry about that being a side effect, i have never seen anyone mention it.

I agree with andyk123 - strengthen those pelvic muscles and do "kegels" - does wonders for the works down there. Basically you work the muscles that help you pee. Google "kegels" and give it a try.