Are you using any PDE5 pills like Viagra? That's generally the first step. Do continue to use the VED (pump) at least for therapy to prevent atrophy. If depending on the VED to achieve and erection and medical grade constriction rings to maintain it, the rings need to be pretty darn tight. This limits you to a maximum length of time for wearing the ring to about 20 minutes. Some guys even wear two constriction rings. Osbon constriction rings are usually considered the best judging from forum posts here on Frank Talk.
If you can achieve an erection unaided then you're way ahead of a lot of cancer survivors who've had their prostate removed, myself included. It sounds like you might be a good candidate for the novelty/sex shop cock rings. See the "Cock Ring" thread in the "Members Only" sub-forum:
https://www.franktalk.org/phpBB3/viewtopic.php?f=16&t=21302. A ring worn on the shaft behind the testicles is best put on before the erection, or at least before it's fully erect. I strongly recommend some manscaping if you haven't already done that. Silicone rings don't get along well with hair. OK, stretch ring with both hands then come up from below and bring it up over the testicles. Then bend you penis down and stretch the ring over it, adjust. Lube may be helpful. Practice makes it easier. I actually did an amateur video on this awhile back but I'm not sure if I still have it of if it's viewable.
Good luck!
PS: Why not try injections?