I have a bunch of Trimax tripods and I use a hold a pole with with 2m rods with 8 min. Vials. Elementary rule in surveying is eliminate/ reduce all potential sources of error... but you're free to continue to measure up your HI with your 12' pocket tape.
It depends on what you are trying to do here. If this is a quick job unrelated to future work by you or others I would get some CORS based solutions and run with them. It doesn't have to be OPUS, I don't know what processor you are using, but I would calculate my own elevations along with OPUS. Looking at where you are you are not all that close to CORS stations, 30 miles to the close one, then 50-70 miles to 3 other ones. That is no problem for OPUS to derive a really good x-y solution but for z your mileage will vary, don't expect it to be better than 3-4 cm, and it may be worse.
If however, you need to continue to build on this project and use as good an elevation as you can, I would find the nearest bench marks with good NAVD88 elevations and tie into them, this way you can find out just what OPUS can do for you and how it relates to Bench Marks in the area. They don't need to be right on top of your project, the beauty of GPS is a point 5-10 miles distance is close by, even 10-15 miles. These don't have to be OPUS solutions, you have a CORS 30 miles away and you can tie to it with a minimum 40 minute session, the ones 50 to 70 miles distance you would want a minimum of 1 hour to 1-1/2 hours. I suspect that you will hit those good marks within 0.20' or less and that should give you confidence to proceed in the future. Once you determine how far "off" OPUS/CORS and NAVD88 are to each other in the area you can decide if you want to hold OPUS or NAVD88. I always use NAVD88 when it's FIRM or DOT (sewer, water, highway, subdivision, city), OPUS/CORS when I'm a hike from Bench Marks and it doesn't matter too much what my basis is (wells, stock ponds, isolated site topos.), when you work in an area often, it pays to develop your own control network and do your own investigation of local conditions.
I can't imagine NOT using fixed height poles and tripods for GPS. If I can eliminate a source of error then I'm going to do so.
