Hi Barney50,
I'm not sure how long you and your OH have been together or how far you've explored together sexually, but there are plenty of things you can try out and explore together over a two year period to keep things interesting and both of you satisfied. PiV is such a small part of everything that can form the collective, diverse concept that is "sexual pleasure".
You can pleasure her with your mouth, fingers, toys (dildos, vibrators, magic wands, etc); you can try sensation play; massage oils, wax, ice cubes, (glass toys are great for hot/cold fun), feathers, bondage/sensory depravation; rope, under-bed restraints, blindfolds, gags, impact play; spanking, padels, floggers, anal play; for her, for you (including pegging, prostate massage, etc).
All these things, while associated with BDSM, doesn't mean you have to dress up in leather & latex and be Dom/sub to enjoy them (unless you want to as part of the fun, of course), you can simply enjoy/explore it as a sensory/pleasure thing.
What I will say about the possibility of including another sexual partner, is that if you've both expressed interest in it as a fetish or sexual desire (her dream, you finding it a turn on), then it's very important that you both communicate that to each other and discuss it at length and find out where it actually stands for you both on the fantasy-reality scale, and, if you want to explore it, do it together, on the same page, from a position of strength and security in your relationship. Approach it as an exploration of joint sexual adventure, and not as a way to fill a perceived void or lack in your relationship.
The most important thing, though, is that you both remain in honest, open communication with each other about your thoughts and feelings. This is important in any relationship of any length at any stage, of course, but particularly for you both over the coming two year period. You need to tell her about your genuine and valid fears about her getting "fed up" with not getting PiV, and she needs to be equally honest and forthright about her own thoughts and feelings. Together, you can ease each others anxieties, and realise that 18 months - 2 years isn't that scary.
Good luck, and have fun!