I'm sorry for your loss.
You are correct that you must have a signed appraisal for any item where you claim a value of more than $5000. You may not split the deduction, you must claim all donations made in 2020 on your 2020 tax return.
I have no idea how to value such an item in used condition. Sometimes, custom items like that are worth very little on the secondary market. Maybe you can take your pictures and other information to a medical supply store or salesperson to get an appraisal. The appraiser does not need to have a formal license, but they must have expertise valuing whatever kind of item you are talking about. The appraiser must also sign a copy of form 8283 listing the donation, and form 8283 must also be signed by a financially responsible official from the charity.
If you list the donations in Turbotax, Turbotax will e-file the form 8283 and then have you print out a copy. You would get that signed by the appraiser and the charity, and then mail it to the IRS after e-filing the rest of your return.
Without those signatures, the most you could claim for the power chair is $5000. You can list the wheelchair, walker and other items separately, the appraisal requirement is not triggered if your total donations are more than $5000, only if any single item is more than $5000.
Full details are in the instructions for form 8283,
https://www.irs.gov/instructions/i8283
